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UNIVERSIDADE FEDERAL RURAL DE PERNAMBUCO
FERNANDO BRUNO VIEIRA DA SILVA
INPUTS OF HEAVY METALS AND RARE EARTH ELEMENTS IN
SOILS VIA PHOSPHATE FERTILIZERS AND CORRECTIVES
Recife
2018
Fernando Bruno Vieira da Silva
Engenheiro Agrônomo
Inputs of heavy metals and rare earth elements in soils via phosphate fertilizers and
correctives
Recife
2018
Tese apresentada ao Programa de Pós-Graduação
em Ciências do Solo, da Universidade Federal Rural
de Pernambuco, como parte dos requisitos para
obtenção do título de Doutor em Agronomia –
Ciências do Solo.
Orientador: Prof. Dr. Clístenes Williams Araújo do
Nascimento
Autorizo a reprodução e divulgação total ou parcial deste trabalho, por qualquer meio
convencional ou eletrônico, para fins de estudo e pesquisa, desde que citada à fonte.
Dados Internacionais de Catalogação na Publicação (CIP)
Sistema Integrado de Bibliotecas da UFRPE
Nome da Biblioteca, Recife-PE, Brasil
S586i Silva, Fernando Bruno Vieira da
Inputs of heavy metals and rare earth elements in soils via
phosphate fertilizers and correctives / Fernando Bruno Vieira da Silva.
– 2018.
85 f. : il.
Orientador: Clístenes Williams Araújo do Nascimento.
Tese (Doutorado) – Universidade Federal Rural de Pernambuco,
Programa de Pós-Graduação em Agronomia, Recife, BR-PE, 2017.
Inclui referências.
1. Contamination 2. Trace elements 3. Agriculture 4. Risk
5. Anthropic activity I. Nascimento, Clístenes Williams Araújo do,
orient. II. Título
CDD 631.4
FERNANDO BRUNO VIEIRA DA SILVA
Inputs of heavy metals and rare earth elements in soils via phosphate fertilizers and
correctives
Tese apresentada ao Programa de Pós-Graduação em Ciências do Solo, da Universidade
Federal Rural de Pernambuco, como parte dos requisitos para obtenção do título de Doutor
em Agronomia – Ciências do Solo.
Aprovada em 08 de janeiro de 2018
_________________________________________________________
Prof. Dr. Clístenes Williams Araújo do Nascimento
Orientador
Universidade Federal Rural de Pernambuco
BANCA EXAMINADORA
_________________________________________________________
Profa. Dra. Karina Patrícia Vieira da Cunha
Universidade Federal do Rio Grande do Norte
_________________________________________________________
Prof. Dr. Edivan Rodrigues de Souza
Universidade Federal Rural de Pernambuco
_________________________________________________________
Prof. Dr. Adailson Pereira de Souza
Universidade Federal da Paraíba
_________________________________________________________
Dra. Josângela do Carmo Trezena de Araújo
Universidade Federal Rural de Pernambuco
SUMMARY
1. GENERAL INTRODUCTION ............................................................................................ 11
1.1 Hypotheses ......................................................................................................................... 12
1.2 General objective ................................................................................................................ 12
1.3 Specific objectives .............................................................................................................. 12
2. BIBLIOGRAPHIC REVIEW ............................................................................................... 15
2.1 The consumption of fertilizers and soil correctives by Brazilian agriculture..................... 15
2.2 Heavy metals and rare earth elements in phosphate fertilizers and soil correctives .......... 17
2.3 Contamination of agricultural soils by heavy metals ......................................................... 20
References ................................................................................................................................ 20
3. ENVIRONMENTAL RISK OF HEAVY METALS IN P-CONTAINING FERTILIZERS
MARKETED IN BRAZIL ....................................................................................................... 25
Abstract ..................................................................................................................................... 25
Resumo ..................................................................................................................................... 26
3.1 Introduction ........................................................................................................................ 27
3.2 Material and Methods ......................................................................................................... 28
3.2.1 Selection, preparation, and analysis of heavy metals in fertilizers ................................. 28
3.2.2 Inputs of heavy metals to soils via fertilizers ................................................................... 29
3.2.3 Statistical analyses .......................................................................................................... 30
3.3 Results and Discussion ....................................................................................................... 30
3.3.1 Trace element concentrations in P-fertilizers ................................................................. 30
3.3.2 Addition of heavy metals in soil via fertilizers ................................................................ 34
3.4 Conclusions ........................................................................................................................ 36
References ................................................................................................................................ 37
4. ASSESSING HEAVY METALS SOURCES IN SUGARCANE BRAZILIAN SOILS: AN
APPROACH USING MULTIVARIATE ANALYSIS ............................................................ 41
Abstract ..................................................................................................................................... 41
Resumo ..................................................................................................................................... 42
4.1 Introduction ........................................................................................................................ 43
4.2 Material and methods ......................................................................................................... 44
4.2.1 Study area and soil sampling .......................................................................................... 44
4.2.2 Soil analysis and quality control ..................................................................................... 47
4.2.3 Statistical Analysis ........................................................................................................... 47
4.3 Results and discussion ........................................................................................................ 48
4.3.1 Descriptive statistics ........................................................................................................ 48
4.3.2 Correlation matrix ........................................................................................................... 49
4.3.3 Principal Component Analysis (PCA), Cluster Analysis (CA) and Discriminant Analysis
(DA) .......................................................................................................................................... 51
4.4 Conclusions ........................................................................................................................ 55
References ................................................................................................................................ 56
5. RARE EARTH ELEMENTS IN SOIL CORRECTIVES AND P-CONTAINING
FERTILIZERS MARKETED IN BRAZIL .............................................................................. 61
Abstract ..................................................................................................................................... 61
Resumo ..................................................................................................................................... 62
5.1 Introduction ........................................................................................................................ 63
5.2 Material and Methods ......................................................................................................... 64
5.2.1 Selection, preparation and total digestion of fertilizers and soil correctives ................. 64
5.2.2 REEs signatures as tracers for P sources ....................................................................... 65
5.2.3 Solubility of rare earth elements in fertilizers ................................................................. 65
5.2.4 Measuring REES and P2O5 total contents ....................................................................... 65
5.2.5 Inputs of REEs into soils through fertilizers and soil correctives ................................... 66
5.2.6 Statistical analyses .......................................................................................................... 67
5.3 Results and Discussion ....................................................................................................... 67
5.3.1 Signatures and concentrations of REEs .......................................................................... 67
5.3.2 REES solubility in P-containing fertilizers ...................................................................... 72
5.3.3 Potential environmental risk and REE additions to Brazilian soils ................................ 74
5.4 Conclusions ........................................................................................................................ 76
References ................................................................................................................................ 77
6. FINAL CONSIDERATIONS ............................................................................................... 83
Inputs of heavy metals and rare earth elements in soils via phosphate fertilizers and
correctives
ABSTRACT
Phosphate fertilizers and corrective are significant sources of soil contamination by
heavy metals and rare earth elements, and long-term cultivated soils can accumulate heavy
metals above the maximum permissible limits. In this sense, the objectives of the study were
(i) to evaluate the potential contamination of Brazilian soils by heavy metals and rare earth via
phosphate fertilizers and corrective; (ii) to evaluate the accumulation of heavy metals in soils
intensively cultivated with sugarcane, as well as to identify the possible sources of
contamination. Initially, were performed total digests of 53 representative samples of P-
containing fertilizers and commercialized in Brazil. Concentrations of As, Cd, Cr, Hg and Pb
were investigated in fertilizers, which are elements regulated by national legislation. Also the
additions of these metals to Brazilian soils were estimated based on the annual consumption
and the average concentrations verified in each product. Subsequently, 60 samples of surface
soils from areas cultivated with sugarcane were collected, these areas had a long history of
use. In the soils, the environmentally available metals (Cd, Cr, Cu, Ni, Pb and Zn) were
evaluated, as well as the physico-chemical characterization. The contents of metals in the soils
were compared with the guiding values of quality; correlated with soil properties and;
subjected to multivariate statistical analyzes to trace the origin of the metals. Finally, the total
and soluble concentrations of Ce, La, Nd, Pr, Sm, Sc, Eu, Y, Dy, Gd, Yb, Er, Tb, Ho and Lu
were investigated in 60 supplies samples (phosphate fertilizer, and limestone) used in
Brazilian agriculture. In addition, the contributions of these elements to Brazilian soils were
estimated through the use of phosphate fertilizers and soil correctives. The results of the study
showed that Cd in phosphate rocks are the most worrisome, and that in the medium term the
heavy metals present in fertilizers containing P do not represent environmental risk. However,
the intensive application of phosphate fertilizers in soils cultivated with sugarcane promoted
the accumulation of Cd above the guiding values of quality, and in some cases the soils
represented a potential risk to human health. With regard to rare earths, it was observed that
the solubility of these elements in phosphate fertilizers is dependent on the raw material used
in industry (igneous or sedimentary). The annual additions of these elements to Brazilian soils
are > 4000 t year-1
and that Ce, La, Nd and Y are the most worrisome. Limestone proved to be
a significant source of Gd for agricultural soils. Therefore, soils that continuously receive high
doses of these inputs are probably enriched with ETRs.
Keywords: Contamination. Trace elements. Agriculture. Risk. Anthropic activity.
Adição de metais pesados e elementos terras raras aos solos via fertilizantes fosfatados e
corretivos
RESUMO
Fertilizantes fosfatados e corretivos são fontes significativas de contaminação do solo
por metais pesados e elementos terras raras, e, solos cultivados por um longo tempo podem
acumular metais pesados acima dos limites máximos permissíveis. Nesse sentido, o estudo
teve por objetivos (i) avaliar o potencial de contaminação dos solos brasileiros por metais
pesados e terras raras via fertilizantes fosfatados e corretivos; (ii) avaliar o acúmulo de metais
pesados em solos intensivamente cultivados com cana-de-açúcar, como também identificar as
possíveis fontes de contaminação. Inicialmente, foram realizadas digestões totais de 53
amostras representativas de fertilizantes contendo P e comercializadas no Brasil. As
concentrações de As, Cd, Cr, Hg e Pb foram investigadas nos fertilizantes, que são os
elementos regulamentados pela legislação nacional. Adicionalmente, as adições destes metais
aos solos brasileiros foram estimadas baseadas no consumo anual e nas concentrações médias
verificadas em cada produto. Posteriormente, 60 amostras de solos superficiais de áreas
cultivadas com cana-de-açúcar foram coletadas, estas áreas tinham um longo histórico de uso.
Nos solos, os teores ambientalmente disponíveis dos metais (Cd, Cr, Cu, Ni, Pb e Zn) foram
avaliados, assim como as caracterizações físico-químicas. Os teores dos metais nos solos
foram comparados com os valores orientadores de qualidade; correlacionados com as
propriedades dos solos e; submetidos às análises estatísticas multivariadas para traçar a
origem dos metais. Por fim, as concentrações totais e solúveis de Ce, La, Nd, Pr, Sm, Sc, Eu,
Y, Dy, Gd, Yb, Er, Tb, Ho e Lu foram investigadas em 60 amostras de insumos (fertilizante
fosfatado, gesso e calcário) utilizados na agricultura brasileira. Além disso, os aportes destes
elementos aos solos brasileiros foram estimados através do uso de fertilizantes fosfatados e
corretivos do solo. Os resultados do estudo demonstraram que Cd em rochas fosfatadas são os
mais preocupantes, e, que em médio prazo os metais pesados presentes nos fertilizantes
contendo P não representam risco ambiental. Contudo, a aplicação intensiva de fertilizantes
fosfatados nos solos cultivados com cana-de-açúcar promoveu o acúmulo de Cd acima dos
valores orientadores de qualidade, e em alguns casos os solos representaram potencial risco à
saúde humana. Com relação aos terras raras, observou-se que a solubilidade destes elementos
nos fertilizantes fosfatados é dependente da matéria prima utilizada na indústria (ígnea ou
sedimentar). As adições anuais destes elementos aos solos brasileiros são > 4000 t ano-1
e que
Ce, La, Nd e Y são os mais preocupantes. O calcário se mostrou uma fonte significativa de
Gd para os solos agrícolas. Desta forma, os solos que recebem continuamente elevadas doses
destes insumos, provavelmente, estão enriquecidos com ETRs.
Palavras-chave: Contaminação. Elementos traço. Agricultura. Risco. Atividade antrópica.
11
1. GENERAL INTRODUCTION
Heavy metals and rare earth elements are harmful to the functionality of the ecosystem
and pose a risk to human health. These elements are found in agricultural supplies, mainly in
phosphate fertilizers and soil correctives, in varying concentrations. The use of these supplies
can promote the enrichment of these elements in soils above the guiding values of quality.
Soils contaminated by metals and rare earth elements are of great public health concern, since
these contaminants can enter the food chain.
Brazil is a major agricultural producer that consumes a large amount of phosphate
fertilizers and soil correctives annually, which represents a significant source of input of
heavy metals and rare earth elements to Brazilian soils. Although there is specific legislation
regulating the concentration of some metals in fertilizers and correctives, there are few studies
that periodically monitor the presence of these elements in these supplies. In addition,
Brazilian legislation is restricted to a limited number of contaminants (As, Cd, Cr, Hg and Pb)
and other hazardous elements present in phosphate and corrective fertilizers, such as rare
earth elements, may represent a potential risk to the functionality of the agroecosystem. The
input of metals and rare earth elements in agriculture via phosphate and corrective fertilizers
is dependent on the concentration of these elements in the supplies, frequency and amount
applied to the soil. In Brazil, there are no surveys that estimate the quantities of metals and
rare earth elements added to the soils by agricultural activities, however, this information
becomes important for future studies that aim to model the balance of mass flow of these
elements in the agricultural scenario, as well as, in the elaboration of public policies to reduce
the additions of metals and rare earth elements to brazilian agriculture.
The sources of contamination of agricultural soils by metals and rare earth elements
are manifold. However, intensively cultivated soils that periodically receive high amounts of
phosphate fertilizers may potentially have heavy metal contents above the maximum
permissible limits. A typical example of this scenario is the sugar-cane monoculture in the
brazilian northeast, as in the case of sugar-alcohol production in the state of Pernambuco. The
cultivation of sugarcane in Pernambuco soils is a historical fact, which began at the time of
the colonization of the country. The crop is extensively cultivated in the state and annually
adds large amounts of phosphate fertilizers and soil correctives. These soils are likely to be
enriched in heavy metals as a result of additions from agricultural activities and other possible
industrial and urban sources. Thus, studies are needed to assess the status of contamination of
12
these soils by heavy metals in order to diagnose possible environmental problems and to
guide the planning of actions capable of preventing and/or remedying soil pollution.
1.1 Hypotheses
The concentrations of heavy metals in P-containing fertilizers marketed in Brazil are
higher than the permissible limits and their use represents a potential risk to the
agroecossystem.
Phosphate fertilizers and soil correctives add significant quantities of rare earth
elements annually to Brazilian agricultural soils.
The rare earth elements contained in the phosphate fertilizers present high solubility.
Soils intensively cultivated with sugarcane are polluted by heavy metals as a result of
the use of high doses of phosphate fertilizers over a long period of time.
1.2 General objective
To evaluate (i) the potential contamination of Brazilian soils by heavy metals (HMs)
and rare earth elements (REEs) via P-containing fertilizers and soil correctives and (ii)
the accumulation of heavy metals in intensively cultivated soils, as well as to identify
possible sources of contamination.
1.3 Specific objectives
To assess the concentrations of As, Cd, Cr, Hg, and Pb in P-containing fertilizers
marketed in Brazil in order to estimate the accumulation of HMs in agricultural soils
through phosphorus fertilizer use.
To estimate the total amount (kg year-1
) and the rate (g ha-1
year-1
) of HMs applied to
soils with aims to calculate the number of mineral fertilizer applications needed to
reach the limit of HMs in agricultural soils established by Brazilian legislation.
To evaluate the contents, the signature and the solubility of REEs (Ce, La, Nd, Pr, Sm,
Sc, Eu, Y, Dy, Gd, Yb, Er, Tb, Ho and Lu) in phosphate fertilizers and soil correctives
marketed in Brazil.
To estimate the amounts of REEs entering soils of Brazil through phosphate fertilizers
and soil correctives.
13
To assess the status of Cd, Cr, Cu, Ni, Pb, and Zn accumulation in soils under long-
term sugarcane cultivation in northeast Brazil.
To trace the possible sources of contamination of agricultural soils through applied
multivariate analysis.
14
15
2. BIBLIOGRAPHIC REVIEW
2.1 The consumption of fertilizers and soil correctives by Brazilian agriculture
Brazil is a country with continental dimensions and it has a wide diversity of soils in
its territory. Most of these soils have limiting factors for agriculture, such as acidity, high
levels of exchangeable Al3+
, very weathered soils presenting low natural fertility, as well as,
low cation exchange capacity and P availability, which are typical characteristics of tropical
soils. Based on these factors, it is necessary to adopt management strategies with the objective
of maximizing the production of the crops grown in these low fertility soils. Thus, the soil
fertilization and correction practices aim to make the soil fertile for the development of
agricultural crops, providing the necessary nutrients for the plants and reducing the soil
acidity. These practices are adopted worldwide, and are based on the need to produce food to
support the rapidly growing population each year (JIAO et al., 2012).
The increasing demand for food production puts a heavy pressure on agricultural
activities, which consequently promote an increase in the consumption of fertilizers and soil
correctives. Studies show that by the end of the 1960s, world fertilizer consumption was 146
million tons, increasing in 2010 to 176 million tons, representing a significant increase in
consumption of these inputs. Brazil is one of the world's largest food producers with 69.2
million hectares of cultivated land (IBGE, 2012; MAPA, 2016). Sugarcane, soybeans and
corn are the main agricultural commodities (FAO, 2017) and fertilizer consumption is
concentrated in four crops: soybean, corn, sugarcane, and coffee; Central-Western region is
the largest consumer (Figure 1). In 2016, the estimated fertilizer consumption was 32.8
million tons, in which 36% were phosphate fertilizers (IPNI, 2017). As a result of such an
intensive agriculture, Brazil is the fourth largest fertilizer consumer behind China, India and
USA (IFA, 2017).
Phosphated rocks are the raw materials used in the manufacture of soluble phosphate
fertilizers. The natural reserves of rock phosphate in Brazil are predominantly igneous
(carbonatite), but the country has the potential to exploit phosphate of sedimentary origin
(phosphorite). Most of the national fertilizer production is concentrated in the Southeast
region, especially in the states of Minas Gerais, São Paulo, and Goiás. The largest active
phosphate mine in the country lies in Catalão, Goiás. Catalão mine is a deposit of dolomitic
carbonate rock as well as the largest national producer of single superphosphate (SSP)
(ABRAM et al., 2011).
16
Figure 1. Brazilian fertilizer consumption by crop and region (IPNI, 2017)
The Brazilian production of phosphate fertilizers using sedimentary ore is low, since
few mines are currently active in the country (TASSINARI; KAHN; RATTI, 2001) (Figure
2). Brazil imports approximately 50% of its consumption in phosphate fertilizers. The
imported amount of phosphate rock and monoammonium phosphate (MAP) in 2014 was 1.7
million tonnes (IFA, 2017), with Morocco being the main supplier of phosphate fertilizers of
sedimentary origin (KRATZ; SCHICK; SCHNUG, 2016; CÁNOVAS et al., 2017).
Brazilian agriculture also consumes a large amount of soil corrective (limestone),
since soils are typically acidic. The consumption of limestone in the 2015/2016 agricultural
harvest was 33.2 million tons, with a higher concentration of use in the South, Southeast and
Midwest regions (ABRACAL, 2017).
17
Figure 2. Spatial distribution of phosphate reserves in Brazil (A), percentage of the phosphate reserve between
the Brazilian states (B) and percentage/origin of phosphate imported for Brazil (C)
Although the use of fertilizers and correctives is essential in agriculture, it is often not
done correctly. The recommendation of the fertilization involves a technical planning that
begins with the representative sampling of the soil, followed by the chemical analysis to
obtain the recommendation made by a specialized professional. However, many producers do
not do this, they usually use pre-defined recommendations without professional assistance.
The use of large amounts of fertilizers over a long period tends to increase the concentration
of an element or substance of environmental interest above the natural content present in the
soil (ROBERTS, 2014). One of the main environmental problems arising from the use of
fertilizers, mainly phosphates, is the increased bioavailability of dangerous elements such as
heavy metals (Cd, Pb, Cr and V) and rare earth elements in soils (JIAO et al., 2012).
2.2 Heavy metals and rare earth elements in phosphate fertilizers and soil correctives
The continued use of phosphates and limestone can also add heavy metals (HMs) and
rare earth elements (REEs) to soil (CAMPOS et al., 2005). The term "heavy metal" is
commonly used to refer to a group of metals and metalloids, which are associated with
environmental contamination and potential toxicity (eg, As, Cd, Cr, Hg, and Pb). Rare earth
elements, in turn, is the denomination given to the group of 17 elements (the Lanthanides
serie - La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu plus Sc and Y)
present in group IIIb of the periodic table that have similar physicochemical properties
(VONCKEN, 2016). Based on their atomic numbers and their masses, REEs are generally
18
divided into two groups: light REEs (from lanthanum - La to europium - Eu) and heavy REES
(from gadolinium - Gd to lutetium - Lu) (DAVRANCHE et al., 2016).
Several studies have evaluated the presence of HMs and REEs in fertilizers and soil
correctives (ABDEL et al., 2001, OTERO et al., 2005, NZIGUHEBA; SMOLDERS, 2008,
FREITAS et al., 2009, LUO et al., 2009). The concentration of such elements in phosphate
fertilizers is dependent on the type of phosphate rock used as raw material in the industry. It is
known that phosphate rocks of sedimentary origin (phosphorite) are richer in HMs than
phosphate rocks of igneous origin (carbonatite) while REEs are more abundant in fertilizers
derived from carbonatites (KRATZ; SCHICK; SCHNUG, 2016; RAMOS et al., 2016).
Kratz, Schick and Schnug (2016), evaluating the HMs content in 68 samples of
sedimentary and igneous phosphate rocks obtained from several parts of the world observed
that the average concentrations of Cd, U and V in the sedimentary sources were at least 110%
higher compared to the igneous sources. The Cd concentrations in phosphate rocks from
Florida (USA) are lower than 100 mg kg-1
, while phosphates derived from African countries
such as Morocco are in the range of 100-350 mg kg-1
(MCLAUGHLIN et al., 1996).
Phosphate rocks from Brazil have low levels of HMs, since 90% of their reserves are of
igneous origin; concentrations of Cd, Cr and Pb in such rocks varied from 4-7, 19-80 and 58-
127 mg kg-1
(CAMPOS et al., 2005; ALCARDE; RODELLA, 2003).
The mean concentrations of Cr, As, Cd, and Pb in phosphate fertilizers marketed in
European countries were 89.5; 7.6; 7.4 and 2.9 mg kg-1
, respectively (NZIGUHEBA;
SMOLDERS, 2008). In China, which is the world's largest fertilizer consumer, HMs
concentrations in phosphate fertilizers were 85.7; 38.5; 15.8 and 0.2 mg kg-1
for Cr, Pb, As,
and Cd, respectively. It was estimated that 1636, 727, 299, and 24 t of Cr, Pb, As and Cd enter
the Chinese soils through phosphate fertilization (LUO et al., 2009).
There are few studies in Brazil that evaluated the concentrations of HMs in phosphate
fertilizers; in addition, most studies were carried out in a small number of samples and mainly
directed at the evaluation of Cd (MENDES et al., 2006; BIZARRO; MEURER; TATSCH,
2008; FREITAS et al., 2009). Mendes et al. (2006) found 45 and 28.0 mg kg-1
of Pb and Cd,
respectively, for triple superphosphate. Bizarro, Meurer and Tatsch (2008) found that Cd
concentrations in phosphate fertilizers were below 17.0 mg kg-1
. Freitas et al. (2009) found
similar Cd concentration (14-15 mg kg-1
) in single (SSP) and triple superphosphate (TSP),
whereas the Pb concentration was highest in SSP samples.
Unlike HMs, REEs are much more abundant in phosphatic deposits of igneous origin.
The REEs concentration in carbonatites may be 150 times higher than phosphorites (RAMOS
19
et al., 2016). Light REEs are higher than heavy REEs in phosphate rocks and, consequently,
in fertilizers. Cerium, La, Pr and Nd are the most enriched elements. Values for Ce, La, Nd,
Pr, and Y vary from 1 - 3934; 7-1926; 2-1529; 5,5 – 447, and 41 – 122 mg kg-1
, respectively
(HU et al., 1998; ABDEL et al., 2001; OTERO et al., 2005; TURRA; FERNANDES;
BACCHI, 2011; WAHEED et al., 2011; RAMOS et al., 2016).
REEs in phosphate fertilizers produced from the main Brazilian phosphate mines
(Catalão - GO, Araxá - MG and Tapira - MG), all derived from carbonatite, were assessed by
Ramos et al. (2016). They found concentrations of 3934 (Ce); 1926 (La); 15 (Nd) and 447
(Pr) mg kg-1
in SSP produced in Catalão; on the other hand, SSP produced in Araxá presented
lower levels (1866, 895, 899 and 235 mg kg-1
for Ce, La, Nd and Pr, respectively). On the
other hand, TSP and MAP produced from the ores of Tapira had levels varying from 809 -
1801; 399-953; 373-814 and 96-237 mg kg-1
for Ce, La, Nd and Pr, respectively. Ramos et al.
(2016) observed that the production of fertilizers more concentrated in P2O5 provided less
accumulation of ETRs. Turra, Fernandes and Bacchi (2011) evaluating the REEs in a set of
phosphate fertilizers, observed the same tendency. Both works attributed the depletion of
REEs in these types of fertilizers to the industrial process of P concentration.
Data on the concentration of HMs and REEs in soil correctives are scarce. Limestone
and mineral gypsum applied in agriculture are ground rock, without any kind of industrial
chemical treatment. Carvalho (2011) found concentrations of HMs in limestone and gypsum
of 10.7 (Cr), 10.5 (Pb) and 5.4 (Cd) mg kg-1
. Turra, Fernandes and Bacchi (2011) found the
following order for REES in limestone: Ce (15) > Nd (7.5) = La (7.5) > Sm (1.3) = Sc (1.3) >
Yb (0.7) > Eu (0.2) = Tb (0.2) > Lu (0.1). Although soil correctives have low HMs and REES
concentrations compared to phosphate fertilizers, the continued application of high doses of
these inputs may be a significant source of these contaminants to soils.
Over a decade ago, Brazil began to regulate As, Cd, Cr, Hg, and Pb levels in fertilizers
and Cd and Pb in soil (Normative Instruction No. 27, Ministry of Agriculture, Livestock and
Supply – MAPA). The maximum permissible levels of HMs were based on human health risk
modeling considering the contribution of HMs to the soil by fertilizers and soil correctives
over time. For P-containing fertilizers, HMs concentration are calculated as a function of the
percentage of P2O5 in the sample. The Regulatory Instruction establishes the following
allowable limits for phosphate fertilizers: 0,05; 2.0; 4.0; 20.0 and 40.0 mg kg-1
/% P2O5 for
Hg, As, Cd, Pb and Cr, respectively. For soil correctives, the allowable limits are 20.0 and
1000.0 mg kg-1
for Cd and Pb, respectively (MAPA, 2006). With regard to REEs, there is no
legislation regulating their levels of agricultural inputs.
20
2.3 Contamination of agricultural soils by heavy metals
Soil contamination by HMs is a subject of great environmental and socioeconomic
interest. This stems from the problems caused to public health and natural ecosystems. Soils
contaminated by HMs represent a high financial cost both to remedy the affected areas and to
treat the exposed population (NAVEEDULLAH et al., 2013; SILVA et al., 2017).
Heavy metals are harmful to human health (NRIAGU, 1988; THOMPSON et al.,
1988). Studies have related the human exposure (acute or chronic) to heavy metals as a major
cause of cardiovascular problems (MYONG et al., 2014), reproductive damages (AL-
GUBORY, 2014), kidney problems (JOHRI; JACQUILLET; UNWIN, 2010), disorders of the
central nervous system (MASON; HARP; HAN, 2014), and cancer (ZHANG et al., 2014;
ITOH et al., 2014).
High concentrations of HMs in the soil are also damaging to the functioning of the
ecosystem. Metals can inhibit plant growth (ARAÚJO; NASCIMENTO, 2010), enzymatic
activities (STRACHEL; WYSZKOWSKA; BACMAGA, 2017), diversity, and structure of
the soil microbial community (JORDANA; PINEDO; BALLESTEROS, 2015).
Heavy metals occur naturally in soil as a result of weathering of parent material (rock)
and in trace amounts (≤ 0.1%, except Fe) (KABATA-PENDIAS, 2011). However, HMs also
enter the soil through mining (SANTOS et al., 2017), municipal waste treatment
(CHARLTON et al., 2016), burning of fossil fuels (WANG et al., 2016), pesticides (LI et al.,
2017), and phosphate fertilizers (KRATZ; SCHICK; SCHNUG, 2016).
In agricultural soils, HMs are mainly added by phosphate fertilization (MICÓ et al.,
2006; PERIS et al., 2007). Continuous applications of high doses of fertilizers can gradually
add HMs to soils beyond permissible limits, such as Cd (JIAO et al., 2012; ROBERTS, 2014).
Crops are an important route of human oral exposure to heavy metals. Therefore, the
monitoring of HMs soils is of great importance for food safety.
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25
3. ENVIRONMENTAL RISK OF HEAVY METALS IN P-CONTAINING
FERTILIZERS MARKETED IN BRAZIL
Abstract
The input of heavy metals (HMs) into agricultural soils from phosphate fertilizers may
compromise the sustainability of agroecosystems and pose a risk to human health. We
evaluated 53 sources of P marketed in Brazil in regard to As, Cd, Cr, Hg, and Pb
concentrations in order to assess the risks associated with their application to soils. Based on
the concentrations of HMs and the consumption of different sources, we estimated the
amounts of HMs added annually to agricultural soils. As, Cr, Pb, and Hg concentrations did
not exceed those found in fertilizers sold in other regions of the world. Phosphate rocks had
Cd concentrations that were twice as high as any other source. The potential risk of Cd
affecting soil quality is at least nine times higher than that of the other HMs assessed. Due to
high consumption, monoammonium phosphates and single superphosphates are the main
sources of HMs to Brazilian soils. In general, HMs in P-containing fertilizers sold in Brazil
did not pose a high human health risk in a medium timeframe. Nevertheless, intensive
application of fertilizers with high concentrations of HMs requires monitoring of the HM
accumulation in soils to ensure the sustainability of agroecosystems.
Key words: Rock phosphate. Agricultural soils. Heavy metals. Cadmium. Arsenic.
26
Risco ambiental de metais pesados em fertilizantes contendo P comercializados no Brasil
Resumo
A adição de metais pesados (MPs) dentro dos solos agrícolas a partir de fertilizantes
fosfatados pode comprometer a sustentabilidade do ecossistema e posar risco a saúde humana.
Analisamos 53 fontes de P comercializados no Brasil no que diz respeito às concentrações de
As, Cd, Cr, Hg e Pb a fim de avaliar os riscos associados com sua aplicação aos solos.
Baseado nas concentrações dos MPs e no consumo das diferentes fontes, estimamos as
quantidades de MPs adicionados anualmente aos solos agrícolas. As concentrações de As, Cr,
Pb e Hg não excederam aquelas encontradas em fertilizantes comercializados em outras
regiões do mundo. Rochas fosfatadas tiveram concentrações de Cd duas vezes superiores a
qualquer outra fonte. O risco potencial do Cd afetar a qualidade do solo foi cerca de nove
vezes maior do que outro MPs avaliado. Devido ao alto consumo, fosfato monoamônio e
superfosfato simples são as principais fontes de MPs aos solos brasileiros. Em geral, MPs nos
fertilizantes contendo P comercializados no Brasil não posam alto risco a saúde humana em
médio prazo. Porém, aplicação intensiva de fertilizantes com concentrações altas de MPs
requer monitoramento do acumulo de MPs em solos para garantir a sustentabilidade do
agroecossistema.
Palavras-chaves: Rocha fosfatada. Solos agrícolas. Metais pesados. Cádmio. Arsênio.
27
3.1 Introduction
Currently, Brazil has 69.2 million hectares of cultivated land and is one of the leading
food producers in the world (IBGE, 2012; MAPA, 2016). As a result of such intensive
agriculture, Brazil is the world's fourth largest fertilizer consumer after China, India, and the
USA (IFA, 2016). In 2015, the estimated consumption of fertilizers was nearly 30 million
tonnes, of which 38% is related to phosphate fertilizers alone (IPNI, 2016).
Although fertilizer use is essential for meeting the crop demand for nutrients exported
from soil, fertilizers can contain heavy metals (HMs) that are inadvertently added to the soil
(KRATZ; SCHICK; SCHNUG, 2016). The amount of HMs transferred to the soil in a single
application of fertilizer is very low or negligible. However, repeated applications of fertilizers
with relatively high contents of HMs can lead to significant accumulation (JIAO et al., 2012).
Therefore, elements such as As, Cd, Cr, Hg, and Pb can affect soil quality and pose a risk to
human and animal health through food chain contamination (HOSSEIWWNPUR;
MOTAGHIAN, 2015).
Heavy metals are present in fertilizers as a result of the composition of raw materials
used in their manufacture. Phosphate rocks and industrial/animal wastes are among the main
sources of HMs in agricultural soils (GONÇALVES JR et al., 2014; SESHADRI; BOLAN;
NAIDU, 2015). The concentration of HMs in nitrogen and potassium fertilizers is usually
very low, and hence, their application does not imply a significant accumulation of HMs
(MCBRIDE; SPIERS, 2001). On the other hand, the prolonged use of P-containing fertilizers
is a major source of HMs in soils, particularly Cd (JIAO et al., 2012). For instance, soils
cultivated with sugarcane and fertilized for a long period with phosphate fertilizers
accumulated Cd levels above the allowable limit according to Brazilian legislation (SILVA et
al., 2016). Estimates of the annual input of HMs in soil via phosphate fertilizers range from
22.0 to 299.0; 24.0 to 30.0; 319.0 to 1626.0; 0.1 to 17.0; and 10.0 to 777.0 tonnes for As, Cd,
Cr, Hg, and Pb, respectively (LUO et al., 2009).
Due to the risk associated with the presence of HMs in fertilizers, several countries
have limited the concentrations of these elements in marketable products (GONÇALVES JR
et al., 2014; JIAO et al., 2012; KRATZ; SCHICK; SCHNUG, 2016). In Brazil, Normative
Instruction No. 27/2006 of the Ministry of Agriculture, Livestock and Supply (MAPA)
established maximum allowable concentrations of HMs in fertilizers (MAPA, 2006).
However, few studies have evaluated the concentration and risk of As, Cd, Cr, Hg, and Pb in
P-containing fertilizers used in Brazil. Cadmium is the most studied element, especially in
28
phosphate rocks (FREITAS et al., 2009). There is also a scarcity of data on the supply of HMs
via fertilizers in Brazilian soils since a plethora of mineral and organic sources are used.
Human and animal health risks associated with HMs in fertilizers need to be evaluated
carefully (HARTLEY et al., 2013). As the concentrations of HMs vary markedly with the raw
materials available in each region or country, the production of local and regional data is
essential. In this scenario, we aimed to assess the concentrations of As, Cd, Cr, Hg, and Pb in
P-containing fertilizers marketed in Brazil in order to estimate the accumulation of HMs in
agricultural soils through phosphorus fertilizer use. In addition, we estimated both the total
amount (kg year-1
) and the rate (g ha-1
year-1
) of HMs applied to soils with aims to calculate
the number of mineral fertilizer applications needed to reach the limit of HMs in agricultural
soils established by Brazilian legislation (MAPA, 2006).
3.2 Material and Methods
3.2.1 Selection, preparation, and analysis of heavy metals in fertilizers
We analyzed 53 P-containing fertilizers sold in Brazil and obtained from the National
Agricultural Laboratory (LANAGRO-PE) of MAPA (Table 1). The samples were dried at 60
°C, pulverized in an agate mortar, and passed through a 0.30-mm mesh stainless steel sieve.
1.000 gram of the various fertilizer powder samples was digested by adding 9 mL
HNO3 + 3 mL HCl in a microwave oven at 175oC for 4 minutes and 30 seconds (USEPA,
1998). This method provides similar results to the official method currently in use in Brazil
Table 1. Description of the phosphate fertilizers evaluated in this study
Product
Number
of
samples
(%)
P2O5a
Consumption
(1,000 t)b
Rock phosphate (RP) 5 24 184.09
Monoammonium phosphate
(MAP) 6 50 3,558.41
Simple superphosphate (SSP) 24 18 5,336.79
Multi-nutrient fertilizers (NPK) 12 13 nd
Organo-mineral + rock phosphate
(OM+RP) 6 26 1,519.27
(a) average value of samples certified by manufacturers; (b) Brazilian consumption in
2015 (ANDA, 2015); nd not available
29
(MAPA, 2006), and it is preferred due to the lower cost of acids, shorter digestion time, and
higher reproducibility (SILVA et al., 2014). All analyses were performed in duplicate.
For quality control, we also analyzed blank samples and a multi-element fertilizer
(SRM 695) with certified HMs concentrations (National Institute of Standards and
Technology, USA). The recoveries of HMs obtained from the certified sample were 97, 100,
72, 90, and 73% for As, Cd, Cr, Hg, and Pb, respectively. Cd, Cr, and Pb concentrations were
determined by optical emission spectrometry (ICP-OES) (Perkin-Elmer/ICP-OES 7000),
while As and Hg concentrations were measured by hydride generation atomic absorption
spectroscopy (HGAAS) (Perkin-Elmer/AA 800).
3.2.2 Inputs of heavy metals to soils via fertilizers
Equation 1 was used to estimate the total annual inputs (tonne year-1
) of HMs in soil
via P-containing fertilizers. Other inputs and losses of HMs in agricultural soils such as
atmospheric deposition, crop uptake or leaching were not included in this calculation.
∑( )
Where TAi is the total amount of the HM (i) added each year (tonne year-1
) from P-containing
fertilizers (j), Qj is the amount of fertilizer (tonne) currently consumed in Brazil, and Cij is the
HM concentration in the fertilizer (mg kg-1
). The rate of application (g ha-1
year-1
) was
estimated with equation 2:
*( ̅
) +
Where TAij is the amount of HM (i) added annually per unit area through fertilizer (j) (g ha-
1), D is the average recommended dose for the main crops grown in Brazil (140 kg ha
-1 P2O5)
(CFSEMG, 1999; IPA, 2008), and FPj is the fraction of P2O5 in fertilizers (0.18, 0.24, 0.26
and 0.50 for SSP, RP, OM+RP and MAP, respectively) (dimensionless).
We also estimated the number of applications needed to increase the HM natural
background of the soil surface layer (0–0.2 m) up to the permissible limit for agricultural soils
established by Brazilian legislation (CONAMA, 2009). The estimate was applied only to
fertilizers with high addition of HMs:
30
Where NAj is the number of fertilizer (j) applications required to increase the HM
concentration (i) in the topsoil up to the allowable limit (AL) (mg kg-1
), and 2.6 x 103 is the
unity conversion factor g ha-1
to mg kg-1
of the parameter TAij, taking into account the depth
of 0.2 m and the soil density of 1300 kg m-3
(FERREIRA, 2010) for the calculation of the soil
mass in 1 hectare.
3.2.3 Statistical analyses
Descriptive statistics of the dataset were obtained by calculating the mean, median,
standard deviation and amplitude. The data with a normal distribution assumption were
verified by the Shapiro-Wilk test (p > 0.05). Data without a normal distribution were
normalized through logarithmic transformation. Hierarchical cluster analysis (AH) was
applied to group different types of P-containing fertilizers. AH was carried out using Ward's
algorithm with the Euclidean distance as a binding measure and the dissimilarity between the
products was assessed as a function of the HM concentration. Statistical analyses were
performed using STATISTICA software (version 10.0).
3.3 Results and Discussion
3.3.1 Trace element concentrations in P-fertilizers
The mean concentration of HMs in fertilizers had the following descending order: Cr
(67.9) > Pb (19.2) > Cd (9.4) > As (6.6) > Hg (0.1). The highest values found for As, Cd, Hg,
and Cr were approximately 7 times higher than their respective median values, and the Pb
concentration was 17 times higher than the median Pb value (Table 2). The distributions of As
and Pb concentrations in the fertilizers had a similar trend, with over 65% of the data lying in
the ranges of 0.5 to 4.2 and 0.3 to 14.7 mg kg-1
, respectively. More than 80% of the Cd and Cr
concentrations were <17.2 mg kg-1
and <148.1 mg kg-1
, respectively. On the other hand, Hg
showed low variability with a high percentage of samples near the mean concentration.
The As concentration found in P-containing fertilizers marketed in Brazil was the
lowest among data from other parts of the world (Table 3). Cr and Pb had high mean
concentrations, but the Cr concentration of the fertilizers marketed in the European Union is
the largest in the literature, as is the Pb concentration in China. The average concentration of
Cd in Brazil, Chile, and Argentina is similar to or higher than those reported for Europe and
31
China. The concentration of Cd in the USA is the highest among the selected countries.
Although there is a scarcity of studies on Hg in fertilizers, Hg concentrations found in
phosphate fertilizers used in China amount to 0.8 mg kg-1
, which is 6 times higher than in
Brazil (LUO et al., 2009).
Three groups of fertilizers regarding HM concentration were distinguished by
hierarchical cluster analysis (Figure 1). In the group of high concentrations, the organic-
mineral fertilizers were the main representatives, with 83% of the evaluated organic-mineral
fertilizers presenting high concentrations of heavy metals. Mixed fertilizers have phosphorus
sources with the highest number of cases (58%) in the group with low concentrations of
heavy metals. This is due to the fact that sources of N and K present very low concentration of
metals (LUO et al., 2009), which causes a dilution effect.
Table 2. Descriptive statistics of the heavy metals content in fertilizers containing P
sold in Brazil
Heavy
metals
Minimum Maximum Mean Median ± SD SW-p
------------------------------ (mg kg-1
) ---------------------------
As 0.54 26.72 6.55 3.41 7.23 <0.001
Cd 0.40 40.03 9.39 6.05 10.08 <0.001
Cr 10.72 341.75 67.82 51.67 77.31 <0.001
Hg 0.06 0.54 0.14 0.11 0.09 <0.001
Pb 0.35 102.46 19.18 6.03 27.64 <0.001
SD standard deviation; SW-p p-value of the Shapiro-Wilk normality test
Table 3. Average content (mg kg-1
) of As, Cd, Cr, Hg and Pb
in phosphate fertilizers marketed in Brazil and in other parts of
the world
Site As Cd Cr Hg Pb
China1
13.5 2.6 na 0.9 30.0
USA (California)2
10.5 53.0 na na na
Europe3
7.6 7.4 89.5 na 2.9
Argentina4
na 10.1 29.7 na 12.2
Chile5
15.1 12.3 na na 10.4
Brazil6
6.6 9.4 67.8 0.1 19.2
na not available; CDFA California Department of Food and Agriculture; 1
LUO et al. (2009); 2 CDFA (1997);
3 Nziguheba and Smolders (2008);
4
Carnelo, Miguez and Marbán (1997); 5 Molina et al. (2009);
6 Present
study.
32
The mean concentrations of As, Hg, and Pb were higher in organic-mineral fertilizers,
while monoammonium phosphate (MAP) and phosphate rock (PR) presented the highest
concentrations of Cr and Cd, respectively (Figure 2). The mean concentration of Pb in
organic-mineral + phosphate rock (OM + PR) was 8 times higher than other sources.
High HM values in the OM + PR can be explained by the use of peat as the main
organic matrix for the production of organic-mineral fertilizers in Brazil (JUNEK et al.,
2014). Peat is organic sediment formed by the decomposition of plant residues in wet and
acidic conditions. Studies have shown that peat soils close to urban and industrial areas can be
enriched by HMs through atmospheric deposition (SHUTTLEWORTH et al., 2014); in
addition, the metal complexation capacity of organic matter makes peat an important Pb
scavenger (FARMER et al., 2005). There have been no studies in Brazil that quantify the
levels of HMs in the peat used for organic-mineral fertilizers; however, data from the
international literature have shown that high concentrations of heavy metals, especially Pb,
can be found in peat (MIGHALL et al., 2002).
Figure 1. Hierarchical clustering of phosphate fertilizers as the contents metals using as criterion the
concentrations of As, Cd, Cr, Hg and Pb in products. SS simple superphosphate, NPK multi-nutrient fertilizers,
RP rock phosphate, OMP organo-mineral + rock phosphate and MP monoammonium phosphate
33
Figure 2. Average contents (standard deviation) of As, Cd, Cr, Hg and Pb in different types of P containing
fertilizers marketed in Brazil. RP phosphate rock, SSP simple superphosphate, MAP monoammonium phosphate
and OMP+RP organo-mineral + rock phosphate
Phosphate rocks presented mean Cd concentrations two times higher than those of
other fertilizers except for MAP. A similar concentration of Cd was found in phosphate rocks
of sedimentary origin marketed in Germany (KRATZ; SCHICK; SCHNUG, 2016), while the
Cd content observed in this study was 4 times higher than in phosphate rocks of igneous
origin (CAMPOS et al., 2005). Phosphate rocks are well known for carrying Cd to soils.
Cadmium concentrations are generally higher in phosphate rocks of sedimentary origin than
those of igneous origin (KRATZ; SCHICK; SCHNUG, 2016). The phosphate deposits in
34
Brazil are predominantly igneous, but a significant amount of this fertilizer is imported from
Morocco, Russia, China, Israel, and the US, which have mainly sedimentary reserves (IFA,
2016); the high Cd concentrations reported here are probably due to these imported levels.
3.3.2 Addition of heavy metals in soil via fertilizers
The average addition rates (g ha-1
year-1
) for the HMs were 4.0 (As), 6.3 (Cd), 47.3
(Cr), 0.08 (Hg), and 16.3 (Pb). The average addition rates for all elements in our study exceed
the average additions of European countries, which were 20.7, 1.0, 1.6 and 2.3 g ha-1
yr-1
for
Cr, Pb, Cd and As, respectively (NZIGUHEBA; SMOLDERS, 2008) (Table 4). The higher
input of heavy metals via fertilizers in Brazilian soils as compared to Europe is mainly due to
the average rate of P applied to the soil. In Brazil, the average recommendation of P2O5 is 140
kg ha-1
, whereas 43 kg ha-1
is the average rate in European countries. The higher demand for P
in Brazilian agriculture as compared to other countries is owing to the high concentration of
Fe and Al oxides in Brazilian soils that adsorb P through a specific adsorption mechanism
(fixation) making it unavailable to plants (FONTES; WEED, 1996). We did not find data on
the addition of Hg via fertilizers for European conditions.
We estimated that phosphate rock adds larger amounts of Cd and Cr than other
fertilizers containing P, while the use of organic-mineral fertilizers contributes greatly to the
input of As and Pb to the soil. Hg additions ranged from 0.04 (MAP) to 0.12 g ha-1
yr-1
(SSP).
The biggest sources of HM are those with the highest levels in their composition except for Cr
(Figure 2). Because the MAP samples are more enriched for P2O5 than phosphate rock, the
amount of MAP needed to achieve the recommended dose is lower than that for phosphate
rock.
Table 4. Mean values (± standard deviation) of per unit area additions (g
ha-1
year-1
) of heavy metals in agricultural soils via phosphate fertilizers
marketed in Brazil
SSP MAP RP OM+RP
As 5.8 (±1.3) 2.4 (±1.1) 1.6 (±0.7) 6.4 (±1.2)
Cd 6.6 (±1.1) 4.8 (±2.0) 11.7 (±3.2) 2.0 (±0.4)
Cr 33.7 (±3.1) 43.9 (±17.8) 75.3 (±20.3) 36.5 (±3.5)
Hg 0.12 (±0.02) 0.04 (±0.01) 0.05 (±0.01) 0.11 (±0.01)
Pb 14.0 (±3.2) 1.1 (±1.0) 16.8 (±10.5) 33.3 (±8.6)
RP phosphate rock, SSP simple superphosphate, MAP monoammonium phosphate and
OMP+RP organo-mineral + rock phosphate
35
Based on the Brazilian consumption and concentrations of HMs in fertilizers, we
estimated that Cr and Pb represent the largest total additions to soil, 915.8 and 209.9 t year-1
,
respectively (Table 5). Minor addition was observed for Hg (1.72 t year-1
). The sources that
incorporate more HMs in soils are SSP and MAP, which are the sources of P most consumed
in Brazil (Table 1). Estimates made with fertilizer composition of HMs indicated that SSP and
MAP together provide more than 90% of Cd, 86% of Cr, and 79% of As. Organic-mineral
fertilizers represented the main input of Pb to soils (45%).
In order to assess whether the levels of HMs in fertilizers exceed the maximum
permitted concentrations, the values of the metals are based on the percentage points of P2O5
established by Brazilian legislation (MAPA, 2006). This practice is also adopted in California
(USA) (AAPFCO, 2015) (Figure 3). Unlike other countries that regulate only Cd in phosphate
fertilizers (CCR, 2007), Brazilian law regulates the concentrations (mg kg-1
%P2O5-1
) of As,
Cd, Cr, Hg, and Pb.
The ascending order of maximum permissible concentrations is Hg < Cd < As < Pb <
Cr. Cd had the highest variability in phosphate fertilizers, with concentrations of 0.33, 0.73
and 1.31 mg kg-1
%P2O5-1
for the median, 75th percentile and maximum values, respectively.
Arsenic and Pb had the highest numbers of anomalous values. SSP samples showed the
highest ratio of As/P2O5, while the highest values of Pb/P2O5 were observed for OM + PR and
SSP samples. When comparing the levels of metals in fertilizers with maximum
concentrations allowed by Brazilian law, it was found that the P sources evaluated meet the
legislative guidelines and are therefore considered safe.
The increases in the concentrations of HMs in soil (mg kg-1
) had the following order:
Cr > Pb > Cd > As > Hg. The Cr and Pb concentrations were an order of magnitude higher
than those of other heavy metals. The increase of Hg through the richest fertilizer (OM + PR)
was negligible, and the estimated number of applications was > 12,000 (Table 6). The
Table 5. Mean values (±standard deviation) of total additions (tonne year-1
) of heavy
metals in agricultural soils via phosphate fertilizers marketed in Brazil
SSP MAP RP OM+RP Total
As 39.7 (±8.9) 30.2(±14.0) 0.5 (±0.2) 18.1 (±8.1) 88.5
Cd 45.6 (±7.5) 61.3 (±25.9) 3.7 (±1.0) 5.6 (±2.9) 116.2
Cr 231.3 (±21.4) 557.7 (±226.7) 23.8 (±6.4) 103.0 (±24.5) 915.8
Hg 0.9 (±0.1) 0.5 (±0.1) 0.02 (±0.00) 0.3 (±0.1) 1.72
Pb 96.0 (±22.2) 14.6 (±12.4) 5.3(±3.3) 94.0 (±59.3) 209.9
RP phosphate rock, SSP simple superphosphate, MAP monoammonium phosphate and OMP+RP
organo-mineral + rock phosphate
36
potential risk for soil sustainability resulting from additions of As, Cr, and Pb via phosphate
fertilizers are for a medium to long-term time frame. If the estimated additions of Cd are kept
high, the concentration in the soil may exceed the allowable limit after 288 applications.
Figure 3. As, Cd, Cr, Hg and Pb concentrations per % of P2O5 in fertilizer containing P sold in Brazil. The
dashed red line shows the permissible limits of heavy metals in phosphate fertilizers in accordance with
Brazilian legislation (MAPA, 2006)
3.4 Conclusions
In general, HMs in P-containing fertilizers sold in Brazil did not pose a human health
risk in a medium timeframe. Nevertheless, intensive application of fertilizers with high
concentrations of HMs requires monitoring of the HM accumulation in soils to ensure the
environmental sustainability of agroecosystems. The contents of all evaluated metals were
Table 6. Number of applications of fertilizers containing P needed to increase the
concentrations of metals in the soil to the permissible limit
Source P2O5 total
(%)
Concentration(a)
(mg kg-1
)
Increase per
application(b)
(mg kg-1
)
Number de
applications
As OM+RP 26 15.0 0.002 6097
Cd RP 24 1.3 0.005 288
Cr RP 24 75.0 0.029 2591
Hg OM+RP 26 0.5 <0.001 12082
Pb OM+RP 26 72.0 0.013 5621
(a) Permissible limit in soil (CONAMA, 2009); (b) based on the application of 140 kg ha-1
of P2O5
incorporated 0.2 m depth in soils with bulk density of 1300 kg m-3
, OM +RP organo-mineral + rock
phosphate, RP rock phosphate.
37
below the maximum found in other regions of the world. Organic-mineral fertilizers are an
important source of As, Hg, and Pb. Monoammonium phosphate and single superphosphates
presented higher additions for As, Cd, and Pb in soils of Brazil. Phosphate rocks pose the
main potential risk for soil quality owing to the high Cd concentration. Mixed fertilizers had
low potential for increasing the concentration of HMs in soils.
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41
4. ASSESSING HEAVY METALS SOURCES IN SUGARCANE BRAZILIAN SOILS:
AN APPROACH USING MULTIVARIATE ANALYSIS
Abstract
Brazil is the world’s largest sugarcane producer and soils in the northeastern part of
the country have been cultivated with the crop for over 450 years. However, so far, there has
been no study on the status of heavy metal accumulation in these long-history cultivated soils.
To fill the gap, we collect soil samples from 60 sugarcane fields in order to determine the
contents of Cd, Cr, Cu, Ni, Pb, and Zn. We used multivariate analysis to distinguish between
natural and anthropogenic sources of these metals in soils. Analytical determinations were
performed in ICP-OES after microwave acid solution digestion. Mean concentrations of Cd,
Cr, Cu, Ni, Pb, and Zn were 1.9, 18.8, 6.4, 4.9, 11.2, and 16.2 mg kg-1
respectively. The
principal component one was associated with lithogenic origin and comprised the metals Cr,
Cu, Ni, and Zn. Cluster analysis confirmed that 68% of the evaluated sites have soil heavy
metal concentrations close to the natural background. The Cd concentration (principal
component two) was clearly associated with anthropogenic sources with P fertilization being
the most likely source of Cd to soils. On the other hand, the third component (Pb
concentration) indicates a mixed origin for this metal (natural and anthropogenic); hence Pb
concentrations are probably related not only to the soil parent material but also to industrial
emissions and urbanization in the vicinity of the agricultural areas.
Key words: Agricultural soils. Soil quality. Trace elements. Cadmium. Lead.
42
Avaliação das fontes de metais pesados em solos brasileiros cultivados com cana-de-
açúcar: uma abordagem usando análise multivariada
Resumo
O Brasil é o maior produtor mundial de cana-de-açúcar e os solos da região nordeste
do país tem sido cultivado com a cultura por mais de 450 anos. Contudo, até momento, não
houve estudo sobre o status do acúmulo de metais pesados nesses solos cultivados por longos
períodos. Para preencher a lacuna, nós coletamos amostras de solos de 60 cultivos de cana-de-
açúcar a fim de determinar os teores de Cd, Cr, Cu, Ni, Pb e Zn. Nós utilizamos analise
multivariada para distinguir entre as fontes naturais e antrópicas destes metais nos solos.
Determinações analíticas foram realizadas em ICP-OES após a digestão em forno micro-
ondas com solução ácida. As concentrações médias de Cd, Cr, Cu, Ni, Pb e Zn foram 1,9;
18,8; 6,4; 4,9; 11,2 e 16,2 mg kg-1
, respectivamente. A componente principal 1 esteve
associada com a origem litogênica e compreendeu os metais Cr, Cu, Ni e Zn. A análise de
agrupamento confirmou que 68% dos locais avaliados tiveram concentrações de metais
pesados nos solos próximo ao background natural. A concentração de Cd (componente
principal 2) foi claramente associada com fontes antropogênicas com a fertilização P sendo a
mais provável fonte de Cd para os solos. Por outro lado, a terceira componente (concentração
de Pb) indicou uma origem mista para este metal (natural e antropogênica); assim as
concentrações de Pb são provavelmente relacionadas não apenas ao material parental do solo
mas também as emissões industriais e urbanização na vizinhança das áreas agrícolas.
Palavras-chaves: Solos agrícolas. Qualidade do solo. Elementos traços. Cádmio. Chumbo.
43
4.1 Introduction
Sugarcane (Saccharum officinarum) is a widely grown crop in Brazil. The sugarcane
industry – including cultivation, processing and refined products – represents an important
segment of the country economy. In 2012, the sugarcane sector contributes US$43.8 billion to
Brazil’s gross domestic product (GDP) – equivalent to almost 2% of the entire Brazilian
economy and higher than the GDP of a European country like Czech Republic (NEVES;
TROMBIM; CONSOLI, 2011). The first sugarcane plantations in the country were
established in Pernambuco state, northeast Brazil, in the 16th century (FREYRE, 1986);
therefore soils in this state have been grown with sugarcane over 450 years. This implies a
long-term application of agrochemicals for promoting crop growth which can contribute to
elevate the concentration of heavy metals in soils (MENDES et al., 2006; KELEPERTZIS,
2014).
Over the last decades there has been an increased interest in soil contamination and its
effects on humans and environment (NAVEEDULLAH et al., 2013; KELEPERTIS, 2014).
Among soil contaminants, heavy metals raise great concern due to their persistence in soils
and potential risks to human health and ecosystems (THOMPSON et al., 1988; ELIK, 2003;
CUI et al., 2004). The natural concentration of heavy metals in soils is generally low and
dependent on the geological parent material; hence, for most metals, anthropogenic sources
are known to dominate over lithogenic contributions (NIU et al., 2013). Anthropogenic inputs
such as sewage irrigation, pesticides and use of fertilizers are the main responsible for the
increase of heavy metals concentrations in agricultural soils (MICÓ et al., 2006; PERIS et al.,
2007; WEI; YANG, 2010). In these soils, the presence of heavy metals deserves careful
monitoring since they can accumulate in less soluble forms or be transferred into soil solution
and consequently deteriorate groundwater and crop quality (SUN et al., 2013; LI et al., 2014;
YANG et al., 2014).
Despite the importance of fertilizers in providing nutrients and ensuring high yields,
long-term repeated application of fertilizers can gradually add potential metal harmful
concentrations in soils (NICHOLSON et al., 2006; LOTTERMOSER, 2009; GRANT;
SHEPPARD, 2008; ROBERTS, 2014; CZARNECHI; DÜRING, 2015). Enrichment of soils
with Cd is a likely result characterizing soils that have suffered a long history of intensive
fertilizer application (MENDES et al., 2006; FREITAS et al., 2009; NIU et al., 2013). Biondi
(2010) reported that Cd was above natural background contents in soils cultivated with
sugarcane in Pernambuco; the author concluded that this resulted from phosphate fertilizer
44
application. Cadmium had the highest pollution index in agricultural soils across China and
the accumulation was also related to fertilization (NIU et al., 2013). The presence of Cd in
phosphate fertilizers is due to its natural occurrence in rocks used for fertilizer manufacturing
(KABATA-PENDIAS, 2011). Many countries, including Brazil, regulate the limits of metal
concentration in fertilizers in order to protect soils and crops against contamination (MAPA,
2006).
The knowledge of anthropogenic versus natural sources of heavy metals in soils is
essential to assess human impact on the environment (PARELHO et al., 2014). Multivariate
analysis has been successfully used to assist the interpretation of environmental data and to
distinguish between natural and anthropogenic inputs (MICÓ et al., 2006). This
differentiation is important to establish soil quality standards regarding heavy metals and to
assess contamination risks of crops and groundwater in areas intensively and long-term
cultivated (MICÓ et al., 2006; NAVEEDULLAH et al., 2013; NIU et al., 2013; SUN et al.,
2013; KELEPERTZIS, 2014).
Nevertheless Brazil is the biggest worldwide sugarcane producer, studies on heavy
metal accumulation in soils are still scarce (CONCEIÇÃO; NAVARRO; SILVA, 2013).
Actually, for the long-history sugarcane plantations of northeast Brazil they are inexistent.
The study was carried out to survey the status of Cd, Cr, Cu, Ni, Pb, and Zn accumulation in
soils under long-term sugarcane cultivation in northeast Brazil. Additionally, we correlated
metal concentrations with physical and chemical soil properties and employed principal
component analysis (PCA), cluster analysis (CA) and discriminant analysis (DA) in order to
distinguish between natural (lithogenic) and anthropogenic sources of such metals in soils and
create groups according to similarity in the soil contamination levels of heavy metals in the
collected areas. To our knowledge this is the first attempt to assess the status of heavy metal
accumulation in soils cultivated with sugarcane for centuries in Brazil.
4.2 Material and methods
4.2.1 Study area and soil sampling
The Pernambuco state, situated in northeast Brazil (Figure 1) has been long associated
with sugarcane plantations. The earliest plantation in operation was in place in 1518 along the
Atlantic coast of the state, but it was not until the 1530’s that sugar agriculture was firmly
established in this region (FREYRE, 1986). The study area is located in the coast zone of
45
Pernambuco and it is characterized by a humid tropical climate with an annual average
temperature of 25.5 °C, annual average rainfall of 2,200 mm and air relative humidity of 80 %
(INMET, 2015). Sugarcane cultivation in this region is characterized by intensive use of
pesticides, limestone and fertilizers owing to the natural acidity and low soil fertility. There
are no other significant human activities in the region that affect soil pollution by metals.
Figure 1. Soil sampling points in areas under sugarcane cultivation and their soil classes along the coastal zone
of Pernambuco state, Brazil
For soil sampling, three of the main sugarcane plantations areas were selected. Haplic
Acrisol is the soil type prevailing in areas 1 and 2 whereas Acric Ferralsol is the soil type
under cultivation in area 3 (Figure 1). Regarding the geology, soils in the study area are
originated from clay loam sediments in the north part and crystalline rocks (rhyolites, granites
and basalts) in the south (Table 1). We collected four topsoil samples (0.0 - 0.2 m depth)
within a 10 m radius from each one of the 60 sampling points; these samples were thoroughly
mixed to form a composite sample to analysis.
46
Table 1. Plane coordinates and geology of the soil sampling sites
Sampling site Coordinates (X – Y)(UTM, meters) Parental material
A1.1 286491 – 9140759
Sandy and sandy-clay sediments
A1.2 286448 – 9140555
A1.3 285494 – 9139524
A1.4 285498 – 9139552
A1.5 284153 – 9139879
A1.6 284169 – 9139880
A1.7 279006 – 9135596
A1.8 279008 – 9135578
A1.9 276192 – 9134495
A1.10 276192 – 9134485
A1.11 286583 – 9144535
A1.12 286583 – 9144518
A1.13 286986 – 9144582
A1.14 286998 – 9144566
A1.15 288819 – 9145841
A1.16 288825 – 9145835
A1.17 288572 – 9146101
A1.18 288582 – 9146109
A1.19 287044 – 9146301
A1.20 287052 – 9146311
A2.1 278059 – 9159665
Sandy and sandy-clay sediments
A2.2 278038 – 9159664
A2.3 277654 – 9157943
A2.4 277649 – 9157945
A2.5 277372 – 9157605
A2.6 277365 – 9157605
A2.7 276384 – 9156361
A2.8 276365 – 9156357
A2.9 274796 – 9155432
A2.10 274790 – 9155454
A2.11 279613 – 9158056
A2.12 279614 – 9158044
A2.13 280562 – 9158268
A2.14 280569 – 9158258
A2.15 281929 – 9158180
A2.16 281828 – 9158161
A2.17 285878 – 9157247
A2.18 282573 – 9157240
A2.19 284434 – 9154542
A2.20 284708 – 9154066
A3.1 263973 – 9053555
Biotite gneiss, mica schistandmigmatite A3.2 263980 – 9053539
A3.3 263768 – 9055544
A3.4 263770 – 9055544
A3.5 260572 – 9058412 Biotite gneissandmigmatite
A3.6 260582 – 9058413
A3.7 264513 – 9050819
Biotite-amphibole granite
A3.8 264526 – 9050814
A3.9 264052 – 9051084
A3.10 264059 – 9051097
A3.11 263651 – 9051051
A3.12 263672 – 9051073
A3.13 269313 – 9047639 Alluvialsediments
A3.14 269289 – 9047624
A3.15 270025 – 9046564 Sandy and sandy-clay sediments
A3.16 270025 – 9046554
A3.17 269959 – 9052596 Rhyolite
A3.18 269958 – 9052602
A3.19 271087 – 9054950 Basalt
A3.20 271094 – 9054954
(A1.n) points samples in area 1, (A2.n) points samples in area 2 and (A3.n) points samples in area 3. (n = 20)
47
4.2.2 Soil analysis and quality control
One kilogram of topsoil (0 – 20 cm) samples were air-dried and sieved through a 2
mm sieve in order to perform routine soil analysis. Soil pH was measured in a 1:2.5 soil:water
suspension. The soil organic matter (SOM) content was determined using the modified
Walkley-Black method (SILVA; TORRADO; JUNIOR, 1999), and the cation exchange
capacity (CEC) was determined by saturation with sodium acetate followed by Na
solubilization with ammonium acetate (MCGEORGE, 1954) and Na determination by flame
photometry. Phosphorus total concentration was measured by colorimetry after microwave
sample digestion (USEPA, 1998). Particle-size distribution was measured by the Bouyoucos
densimeter method modified by Almeida (2008).
For metals determination, 1.0 g soil was digested in Teflon vessels with 9.0 mL of
HNO3 and 3.0 mL of HCl in a microwave oven (USEPA, 1998). All extracts were transferred
to 50-mL certified flasks (NBR ISO/IEC) filled with ultrapure water and filtered in a slow
filter paper. High purity acids were used in the analysis (Merck PA). Glassware was cleaned
and decontaminated in a 5 % nitric acid solution for 24 h and then rinsed with distilled water.
The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were determined by inductively coupled
plasma (ICP-OES). Analytical data quality and standard operation procedures, such as curve
recalibration, analysis of reagent blanks, limit of quantification calculation (LOQ) and
analysis of standard reference material (NIST 2710 - Montana Soil) were carried out. NIST
recoveries were 113% (Cd), 92% (Cr), 89% (Cu), 100% (Ni), 101% (Pb), and 85% (Zn). The
LOQ calculated were 0.0002, 0.0007, 0.003, 0.0012, 0.0042, and 0.0006 mg L-1
for Cd, Cr,
Cu, Ni, Pb e Zn, respectively.
4.2.3 Statistical Analysis
Descriptive statistics (mean, minimum and maximum values, and standard deviation)
for raw soil data were established and Shapiro-Wilk distribution test used for data normality
assessment. A p value higher than 0.05 was used to agree with the hypothesis of the data set
belongs to a normal distribution. Multivariate analyses were performed with STATISTICA
(v. 10) software. The concentrations of metals were submitted to principal component
analysis (PCA) and interpreted according to their natural, anthropogenic or mixed sources.
Varimax rotation was applied since orthogonal rotation minimizes the number of variables
with high loading on each component and makes data interpretation easier (MICÓ et al.,
48
2006). Cluster Analysis with adoption Cd, Cr, Cu, Ni, Pb and Zn content as dissimilarity
standard was applied to the areas by Ward's method and use of Euclidean distance. The
validation of the cluster analysis was accomplished with discriminant analysis. Pearson’s
correlation was used to relate soil properties and heavy metal concentrations.
4.3 Results and discussion
4.3.1 Descriptive statistics
Soil chemical and physical variables did not present normal distribution according to
Shapiro-Wilk distribution test (p < 0.05), except for pH (Table 2). The pH values varied from
4.5 to 6.7 and more than one-half of the samples presented a very acidic pH (< 5.5), typical of
tropical soils. Soil organic matter (SOM) ranged from 11.1 to 94.4 g kg-1
with an average
value of 25.9 g kg-1
. Most of the samples analyzed are coarse texture soils with mean values
of 674.0 g kg-1
of sand. This is the main reason for the low cation exchange capacity (CEC) of
these soils. The values of total P broadly varied from 9.0 to 1118.0 mg kg-1
with most of the
soil samples (80%) presenting P concentrations between 100 and 400 mg kg-1
. Such results
indicate the intensive use of fertilizers that can build P levels in soil higher than sugarcane
requirement and add toxic metals to soil system.
Table 2. Descriptive statistic for soil chemical and physical
characteristics and heavy metal concentrations
Mean Min
Max
SD
S-W
test
pH 5.6 4.5 6.7 0.6 0.40
SOM (g kg-1
) 25.9 11.1 94.4 13.3 0.00
CEC (cmolcdm-3
) 11.7 0.2 107.0 27.7 0.00
Ptotal(mg kg-1
) 275.8 9.4 1118.5 239.0 0.00
Sand (g kg-1
) 674.8 174.5 956.7 204.8 0.00
Clay (g kg-1
) 238.8 22.0 678.3 152.2 0.00
Cd (mg kg-1
) 1.9 1.0 4.3 0.8 0.00
Cr (mg kg-1
) 18.8 0.2 111.5 22.8 0.00
Cu (mg kg-1
) 6.4 <LOQ
36.4 8.0 0.00
Ni (mg kg-1
) 4.9 <LOQ
41.2 7.4 0.00
Pb (mg kg-1
) 11.2 1.1 32.3 6.7 0.01
Zn (mg kg-1
) 16.2 2.1 58.5 10.6 0.00 SOM soil organic matter, CEC cation exchange capacity, Ptotal total phosphorus in soil,
Min minimum value, Max maximum value, SD standard deviation, S-W test value p in
Shapiro-Wilk test, LOQ limite of quantifitation (0.003 and 0.0012 mg kg-1
for Cu and
Ni, respectively)
49
Heavy metal concentrations in soils followed the order Cr > Zn > Pb > Cu > Ni > Cd
(Table 2). All heavy metals presented concentrations above the quality reference values
(QRVs) for soils of the state (CPRH 2014) in at least three of the surveyed areas (Figure 2).
This clearly suggests that anthropogenic inputs have increased metal concentrations in such
soils to levels exceeding the lithogenic contribution through weathering. Significant
increments in metal concentration in agricultural soils have been reported in several parts of
the world (MENDES et al., 2006; PERIS et al., 2007; SUN et al., 2013; KELEPERTZIS,
2014; PARELHO et al., 2014).
Cadmium concentrations were higher than QRV (0.5 mg kg-1
) in all the 60 soil
samples, with the mean value for the element being three times higher than the expected
background content (Figure 2). Based on the national regulation regarding heavy metal
concentrations in agricultural soils (CONAMA, 2009), six sugarcane fields can pose an
unacceptable risk to human health since their Cd concentrations are above 3.0 mg kg-1
. This
implies that such contaminated areas must pass through a site-specific risk assessment to
determine the need and urgency of remediation. Soils contaminated with heavy metals in an
agricultural scenario can pose a threat to human health mainly through soil ingestion, crop
consumption, inhalation of soil particles and dermal uptake via soil (SWARTJES et al.,
2012). Copper and Pb concentrations exceeded their QRVs in 37 % of the areas hence
anthropogenic influence can be at least partially related to such an increase. Only a few soil
samples have Zn, Ni, and Cr concentrations above their region’s natural occurrence, although
two of these samples are probably contaminated owing to agrochemical inputs.
4.3.2 Correlation matrix
Soil organic matter (SOM) was positively correlated with Cd, Cr and Cu (Table 3) in
spite of the low SOM mean value found in soils (Table 2). It is well-known that organic
matter strongly binds Cr3+
e Cu2+
while presents a moderate binding capacity for Cd+2
(BLUME; BRÜMER, 1987; KABATA-PENDIAS, 2011).
The clay content exhibited significant correlations with all the metals (Table 3). Clay
minerals are probably the main soil colloids regarding heavy metals adsorption in the studied
soils owing to the low contents of soil organic matter. Given the low correlation between
metals and CEC, it is likely that metals are bound mostly by covalence on kaolinite and iron
and aluminum oxides (ARAÚJO et al., 2002). The adsorption mechanisms in these clay
50
minerals take place by high energy covalent binding (specific adsorption), a common reaction
between soil oxides and metallic ions (NAIDU; SUMNER; HARTER, 1998).
Figure 2. Heavy metal concentrations in sugar cane fields as compared to regional soil quality reference values
(QRVs) for metals (CPRH, 2014)
A positive and significant correlation was obtained between total P concentration and
metals studied (Table 3) with highest values for Cu as also reported by Kelepertzis (2014).
The role P plays in the metal sorption in soils is due to formation of low solubility precipitates
resulting from the reaction between free phosphate ions (HPO42-
) with metallic cations (Cr3+
,
Cu2+
, Cd2+
, Pb2+
, Ni2+
e Zn2+
) in soil solution (CRANNELL et al., 2000). For instance,
51
formation of pyromorphite [Pb5(PO4)3(Cl, Br, OH e F)], a low solubility lead phosphate can
take place through P amendment to Pb contaminated soils which reduces Pb bioavailability
(NIAGRU, 1984; MA, 1996). Additionally, as HPO42-
is adsorbed on positively charged
surfaces of iron oxides, aluminum oxides and phyllosilicates the net superficial charge
changes from positive to negative. This increases the cation exchange capacity (CEC) and
promotes the formation of oxide-phosphate-metal type complexes (GUILHERME;
ANDERSON, 1998; OLIVEIRA et al., 2010).
4.3.3 Principal Component Analysis (PCA), Cluster Analysis (CA) and Discriminant Analysis
(DA)
Given the long-term soils cultivated with sugarcane are clearly enriched in some heavy
metals compared to the natural background of the region, PCA was used to distinguish
between natural and anthropogenic sources of these elements in soils. The eigenvalues of the
first two extracted factors were greater than 1.0 (Table 4); thus heavy metals can be grouped
into a three-component model that accounts for 91 % of the total variance. The spatial
representation displaying the metals association of the three rotated components is shown in
Figure 3. The component matrix showed that Cr, Cu, Ni, and Zn were associated with the first
component (PC1); the second component (PC2) included Cd while Pb showed a high loading
in the third component (PC3).
The principal component PC1 including Cr, Cu, Ni, and Zn explained 64.2% of the
total variance (Table 4). The variability of these elements in soils seems to be related to a
common source, in such a case the underlying soil parent material. Indeed, most of the studied
Table 3. Correlation matrix between heavy
metal concentrations and selected soil
properties
SOM
Clay CEC
Ptotal
Cd 0.35* 0.67** 0.34* 0.35*
Cr 0.35* 0.67** 0.24ns
0.41**
Cu 0.29* 0.34* 0.38** 0.67**
Ni 0.16ns
0.46** 0.28* 0.37**
Pb 0.09ns
0.44** 0.23ns
0.32*
Zn 0.14ns
0.35* 0.28ns
0.43**
SOM soil organic matter, CECcation exchange
capacity,Ptotaltotal phosphorus in soil, * p < 0.05, ** p <
0.01, ns
not significant.
52
areas showed concentrations of Cr, Cu, Ni, and Zn very similar to those regionally found in
natural background condition (BIONDI et al., 2011).
Figure 3. Loading plots of the three components influencing heavy metals variation in soils cultivated with sugarcane
The principal component PC2 includes Cd only. This component is defined as
anthropogenic and related to agriculture practices, chiefly fertilization. As mentioned
previously, all the soil samples presented Cd concentrations higher than the expected natural
content of the element in soils. This confirms the interpretation of Cd anthropogenic origin.
Phosphate fertilizers are a well-known source of Cd in agricultural soils (MENDES et al.,
Table 4. Principal component analysis matrix for heavy metal
concentrations in soils grown with sugarcane in Pernambuco state,
Brazil. Significant loading factors are underlined
Components Eigenvalues % Total
Variance
%
CummulativeVariance
1 3.85 64.26 64.26
2 1.01 16.52 81.08
3 0.59 9.90 90.99
Element Component1 Component2 Component3
Cd 0.16 0.95 0.16
Cr 0.82 0.42 0.19
Cu 0.89 0.25 0.13
Ni 0.82 -0.26 0.38
Pb 0.30 0.20 0.92
Zn 0.80 0.22 0.35
53
2006; NICHOLSON et al., 2006; FREITAS et al., 2009; CARVALHO; NASCIMENTO;
BIONDI, 2012; NAVEEDULLAH et al., 2013; YANG et al., 2014) and the increase of Cd
concentration is closely related to the length of time soils have been fertilized
(KELEPERTZIS, 2014; PARELHO et al., 2014). Therefore, the long history of sugarcane
cultivation along with the use of fertilizers resulted in extensive Cd accumulation in soils
from Pernambuco state. However, other sources such as atmospheric deposition, animal
manures and sewage sludge applied to soils can also contribute to increased concentrations of
Cd in agricultural soils (MICÓ et al., 2006).
PC3 includes Pb and explained only 9.9 % of total variance but it was clearly
differentiated from the other components; this is apparently an indication of mixed sources
contributing to Pb addition in soils. Lead is generally linked to atmosphere deposition from
vehicles emissions and has been widely used in chemical, metallurgical and siderurgical
industries (KABATA-PENDIAS, 2011; HU et al., 2013). The highest Pb concentrations were
found in soils from the south coast of Pernambuco (Figure 1). This region has a port and
industrial complex established in 1978 which comprises several potentially polluting
activities, including a petrochemical industry regarded as a major source of Pb contamination
to atmosphere (SCHRODER et al., 2000). Thus atmospheric deposition of Pb originated from
these industrial sources could explain the above background concentrations of this metal in
soils.
The hierarchical cluster analysis (Figure 4) allowed the definition of four geochemical
groups which dissimilarity was validated through a discriminant analysis with 98.3 % of total
certainty (Table 5). When heavy metals are used as a dissimilarity criterion, there is area
intersection for groups 1 and 2 only; these groups present the lowest concentrations of Cr, Cu,
Ni, and Pb in soil. The similarity between these two groups (G1 and G2) is indicated by
proximity of their centroids in comparison with groups 3 and 4 (Figure 5).
The majority of soil samples assessed (68 %) are in group 1 (Figure 4). These samples
presented Cr, Cu, Ni, Pb and Zn concentration in the normal range (≤ QRVs) and are related
to soil parent material. A closer scrutiny of group 2 reveals that concentrations of Cd and Cu
in the 12 soil samples of this group are above their respective quality reference values while
Cr and Pb soil concentrations are higher than the QRV just for some samples. Group 3
includes four soil samples (cases 9, 41, 44, and 53) that presented Cd, Cr, Cu, Ni, and Pb
concentrations above background levels. The two soil samples in group 4 (59 and 60) are
characterized by the highest concentrations of Cd (4.3 mg kg-1
), Cr (110.9 mg kg-1
), Cu (35.2
mg kg-1
), Ni (40.3 mg kg-1
), and Zn (57.9 mg kg-1
). For instance, Zn and Cu concentrations in
54
soils are 2 to 7-fold higher than their QRVs and Cd concentration is 43 % higher than the
permissible concentration for Brazilian agricultural soils (CONAMA, 2009).
Figure. 4. Dendrogram obtain by hierarchical cluter analysis for heavy metals contents in soil samples.
Table 5. Confusion matrix among geochemical groups
formed throughhierarchical cluster analysis
G1 G2 G3 G4 Total Certainty
%
G1 41 0 0 0 41 100
G2 1 12 0 0 13 92
G3 0 0 4 0 4 100
G4 0 0 0 2 2 100
Total 42 12 4 2 60 98
55
Figure 5. Function discriminant analysis plot for four geochemical groups in soil cultivated with sugar cane
4.4 Conclusions
Agricultural soils have been contaminated by heavy metals in several countries. The
results presented here increase our knowledge of the accumulation and sources of heavy
metals in soils under long-term sugarcane cultivation in Northeast Brazil. The PCA enabled to
distinguish between natural and anthropogenic sources of metals in soils. The principal
component one (PC1) was associated with lithogenic origin and comprised the metals Cr, Cu,
Ni, and Zn. Indeed, most concentrations of these metals were close to the natural background
concentrations for soils of the region. The Cd concentrations (PC2) were clearly associated
with anthropogenic sources as a consequence of P fertilization and other amendments to soil.
All the soil samples analyzed presented Cd concentrations above the state’s soil quality
standard; six out of 60 sugarcane fields pose an risk to human health since their Cd
concentrations are above 3.0 mg kg-1
. Finally, Pb concentration (PC3) indicates a mixed
origin for this metal (natural and anthropogenic); hence Pb concentrations are probably
related not only to the soil parent material but also to industrial emissions and urbanization in
the vicinity of the agricultural areas. The cluster and discriminant analysis allowed the
identification of four geochemical groups regarding different levels of soil contamination;
most values of Cr, Cu, Ni, Pb and Zn (68%) were in the range of soil background
G1
G2
G3
G4
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
35
-10 -5 0 5 10 15 20 25 30
Fu
nct
ion
(6
,62
%)
Function (92,49 %)
Group 1 Group 2 Group 3 Group 4 Group centroid
56
concentrations while 10% of soil samples were grouped into the more metal enriched soils for
Cd, Cr and Ni. Our results are consistent with other reports on accumulation of metals in soils
that demonstrate the potential of agriculture to increase the heavy metal concentrations above
regulatory standards. The data represent the first survey of metal concentrations in sugarcane
soils of Northeast Brazil and highlight the importance of monitoring the concentrations of
heavy metals in soils intensively cultivated in order to protect the soil ecological functions
and the human health.
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máximas admitidas para agentes fitotóxicos, patogênicos ao homem, animais e plantas,
metais pesados, pragas e ervas daninhas". Dísponivel em:
<http://agrolab.com.br/documento/13>. Acessado em 11 junho 2014.
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61
5. RARE EARTH ELEMENTS IN SOIL CORRECTIVES AND P-CONTAINING
FERTILIZERS MARKETED IN BRAZIL
Abstract
The mineral exploration of rare earth elements (REEs) and their entry into the soil by
pollution or fertilizer application has generated concern about possible environmental impacts
and human health. In this sense, we evaluated 60 samples of agricultural inputs (limestone,
gypsum and phosphate fertilizers) marketed in Brazil in order to characterize their contents,
signature and solubility of REEs. The fertilizers from igneous origin presenting the largest
accumulation of REEs. Regardless of origin, the accumulations of the light REEs Ce, La, Nd,
Pr, Sm and Eu were larger than the heavy ones (Y, Dy, Gd, Er, Yb, Ho, Tb and Lu). The
solubility of fertilizers produced from sedimentary sources was greater than that of igneous
sources. The mean annual REEs contribution of SSP and organo-mineral + phosphate rock
(both of igneous origin) to soils was > 4000 t year-1
and the highest additions were for Ce, La,
Nd and Y. The limestone contributed 250 t of Gd, more than any other source. Thus,
phosphate fertilization and liming were considered to be significant sources of REEs and soils
receiving continuously high doses of these inputs are likely to be enriched in REEs.
Key words: Rare earth. Phosphate fertilizers. Agricultural soil. Contamination.
62
Elementos terras raras em corretivos do solo e fertilizantes contendo P comercializado
no Brasil
Resumo
A exploração mineral de elementos terras raras (ETRs) e sua entrada no solo por
poluição ou aplicação de fertilizantes, tem gerado preocupação quanto aos possíveis impactos
ambientais e na saúde humana. Neste sentido, nós avaliamos 60 amostras de insumos
agrícolas (calcário, gesso e fertilizantes fosfatados) comercializados no Brasil a fim de
caracterizar seus teores, assinatura e solubilidade de elementos terras raras. Os fertilizantes de
origem ígnea apresentaram o acúmulo de ETRs. Independentemente da origem, os acúmulos
dos ETRs leves Ce, La, Nd, Pr, Sm e Eu foram maiores do que dos pesados (Y, Dy, Gd, Er,
Yb, Ho, Tb e Lu). A solubilidade dos fertilizantes produzidos de fontes sedimentares foram
maiores do que das fontes ígneas. O aporte médio anual de ETRs destas fontes para os solos
foi > 4000 t ano-1
e as maiores adições foram observadas para Ce, La, Nd e Y. O calcário
aportou 250 t ano-1
de Gd, mais do que qualquer outra fonte. Deste modo, a adubação
fosfatada e a calagem foram consideradas fontes significativas de ETRs e os solos que
recebem continuamente elevadas doses destes insumos, provavelmente, são enriquecidos em
ETRs.
Palavras-chaves: Terra rara. Fertilizante fosfatado. Solo agrícola. Contaminação.
63
5.1 Introduction
Rare earth elements (REEs) are a group of 17 elements, comprising the 15 lanthanides
(lanthanum - La, cerium - Ce, praseodymium - Pr, neodymium - Nd, promethium - Pm,
samarium - Sm, europium - Eu, gadolinium - Gd, terbium - Tb, dysprosium - Dy, holmium -
Ho, erbium - Er, thulium - Tm, ytterbium - Yb, lutetium - Lu), as well as scandium (Sc) and
yttrium (Y). REEs have similar chemical and physical properties (VONCKEN, 2016) and are
commonly found together in geological deposits of phosphate rocks (RAMOS et al., 2016).
In recent years, the increasing anthropogenic contribution of REEs to the environment
through mining activities (LIANG; LI; WANG, 2014; ATIBU et al., 2018), the disposal of
industrial effluents (HE et al., 2010) and the extensive application of REEs enriched fertilizers
(CAO et al., 2001; PANG; LI; PENG, 2002), has generated concern about possible
environmental impacts and human health. Studies have correlated environmental
contamination by REEs with diseases in humans, occupational poisoning, water and soil
pollution, as well as the reduction in soil macro and microfauna diversity (MARUBASHI;
HIRAMO; SUZUKI, 1998; LI et al., 2013; RIM; KOO; PARK, 2013).
Rare earth elements are mainly incorporeted in agricultural soils by phosphate
fertilization (HU et al., 2006). The concents of REEs in nitrogen and potassium fertilizers, as
well as in soil correctives are generally lower than those found in phosphates (TURRA;
FERNANDES; BACCHI, 2011). However, the continuous application of high amounts of soil
correctives may imply in significant amounts of REEs added to soil (ABDEL et al., 2001).
Phosphate rock is the raw material for the manufacture of phosphate fertilizers and is also
widely used in natura as a source of phosphorus. Phosphates naturally contain REEs, which
may be of concern for environmental contamination (HEIN et al., 2016; POLETTI et al.,
2016; RAMOS et al., 2016; XIN et al., 2016; CERVA-ALVES et al., 2017; GARNIT;
BOUHLEL; JARVIS, 2017) as significant amounts of REEs remain in the fertilizer after the
production (RAMOS et al., 2016).
Phosphate fertilizers are mainly derived from phosphorite (sedimentary rock) and
carbonatite (igneous rock). Fertilizers containing P derived from carbonatite are generally
enriched in REEs (OTERO et al., 2005). The Brazilian deposits of phosphates are mainly
from carbonatite origin. However, Brazil imports large amounts of ore and phosphate
fertilizers (IFA, 2016), making it difficult to track the origin of commercial fertilizers. As a
solution to this problem, the work of Otero et al. (2005) and Ramos et al. (2016) have shown
that REEs can be used as tracers of origin (igneous or sedimentary) of the raw materials used
64
in the manufacture of phosphate fertilizers. A common method for this purpose is the
comparison of the distribution patterns (signatures) of the REEs in the samples and in the
possible sources. For this, the normalized REE contents are graphically plotted; in this graph
the content of each REE is divided by the content of the same REE in a reference material
(normalization) (PIPER; BAU, 2013).
China is the world's largest producer and consumer of REEs; fertilizers enriched in
REEs (containing on average 19.8, 5.4, 4.6, 1.9, 0.4, and 0.1% of La2O3, Nd2O3, CeO2,
Pr6O11, Sm2O3 and Eu2O3, respectively) have been produced for several decades and
extensively applied in Chinese agriculture as it is believed that these elements provide greater
crop productivity. In Brazil, on the other hand, there is no fertilization recommendation and
no legislation to regulate REE reference values in fertilizers and soil correctives. There are
few studies in the country that investigated the REEs levels in agricultural inputs (RAMOS et
al., 2006; TURRA; FERNANDES; BACCHI, 2011); there is also little information on the
uptake of these elements by plants in soils fertilized with different phosphate fertilizers and
their long term accumulation in the soil. Thus, the determination of the total and soluble
contents of REEs in fertilizers and correctives is important to monitor the risks of
contamination and pollution of these elements in soils.
Brazil is currently the 4th largest consumer of fertilizers in the world (IFA, 2016).
Phosphate fertilizers and soil corrective are used intensively in Brazilian agriculture; in 2016,
the country consumed 11.8 and 33.0 million tons of phosphate fertilizers and limestone,
respectively (ABRACAL, 2017; IPNI, 2017). These fertilizers and correctives are an
important route of entry of REEs to the soil. In this context, the present work evaluated the
contents, the signature and the solubility of REEs in phosphate fertilizers and soil correctives
marketed in Brazil. In addition, we estimated the amounts of REEs entering soils of Brazil
through these inputs.
5.2 Material and Methods
5.2.1 Selection, preparation and total digestion of fertilizers and soil correctives
We selected 60 samples of fertilizers and soil correctives as follows: limestone (20),
gypsum (10), single superphosphate - SSP (12), triple superphosphate - TSP (2),
monoammonium phosphate – MAP (6), organo-mineral + phosphate rock (6) and phosphate
rock (4). Samples were obtained from the National Agricultural Laboratory (LANAGRO-PE)
65
of the Ministry of Agriculture, Livestock and Supply (MAPA). The samples were oven dried
(65 ° C), powdered in agate mortar and sieved in a 0.3 mm stainless steel mesh.
Samples of 1.000 g of each product were digested at 250 ° C with HF, HClO4 and
HNO3 (2: 1: 1) in teflon beakers (ESTEVEZ et al., 2001). After digestion, the extracts were
filtered (Ø < 2 μm) and the volume measured in 25 mL volumetric flasks with ultrapure
water.
5.2.2 REEs signatures as tracers for P sources
The fertilizer samples were grouped according to their REE signatures. The content of
each REE in each sample was divided by its content into a reference material, for example:
[ ]
[ ]
The reference material used in the present study was Post Archaen Australian Shale
(PAAS) (MCLENNAN, 2001). PAAS has been used in other works with the same objective
(OTERO et al., 2005; SANDERS et al., 2013). Data were collected in the literature on the
mean REE levels in carbonatites (POLETTI et al., 2016; TROFANENKO et al., 2016;
CERVA-ALVES et al., 2017) and phosphorites (HEIN et al., 2016; KHATER et al., 2016;
XIN et al., 2016; GARNIT; BOUHLEL; JARVIS, 2017), which were also normalized by
PAAS. The source of P in the fertilizers was traced by the similarity between the REE
signatures in the samples with those of the possible sources (carbonatite or phosphorite).
5.2.3 Solubility of rare earth elements in fertilizers
Samples of 1.000 g of each fertilizer were agitated with 100 mL of 2 % citric acid
solution, a standard extractor for analysis of phosphate fertilizers in Brazil (MAPA, 2013) and
then filtered. The solubility of the elements was expressed as a percentage relative to the total
contents, as follows:
(
)
Where % Solij is the soluble percentage of REE (i) in the phosphate fertilizer (j), while
CSolij and Cij are the soluble and total concentrations (mg kg-1
), respectively.
5.2.4 Measuring REES and P2O5 total contents
66
The contents of the REEs Ce, La, Nd, Pr, Sm, Sc, Eu, Y, Dy, Gd, Yb, Er, Tb, Ho and
Lu in the extracts were determined by coupled plasma optical emission spectrometry (ICP -
OES/Optima 7000 Perkin Elmer). The total P contents in the fertilizers were determined by
photocolorimetry. Standard procedures for quality control such as recalibration curve, blank
sample analysis and reference material SRM 2709 (San Joaquin Soil, NIST - National
Institute of Standards and Technology) were used. The recoveries of Ce, La, Nd, Sm, Sc, Dy
and Gd ranged from 70 to 98%. All analyzes were performed in duplicate.
5.2.5 Inputs of REEs into soils through fertilizers and soil correctives
Equation 2 was used to estimate the total annual addition (ton ha-1
) of REEs in soils
through the application of limestone, gypsum and P-containing fertilizers. Other sources of
addition and the possible losses of these elements in the soil were not considered in the
calculation, since they did not contribute in a significant way to the mass balance of these
elements in the soils.
∑( )
Where TAi is the total amount of REE (i) added annually (ton ha-1
) from soil correctives and
phosphate fertilizers (j), Qj is the amount of the input (ton) currently consumed in Brazil and
Cij is the REE (i) in the soil correctives and phosphate fertilizers (j).
The addition per unit area (g ha-1
year-1
) was estimated according to equation 3:
*( ̅
) +
Where TAij(a) is the amount of REE (i) added annually per unit area through fertilizers
containing P (g ha-1
), except phosphate rock, D is the recommended average dose for the main
crops grown in Brazil (140 kg ha-1
P2O5) (CFSEMG, 1999; IPA, 2008) and FPj is the fraction
of P2O5 in fertilizers containing P (0.18, 0.26, 0.40 and 0.50 for SSP, OM + RF, TSP and
MAP , respectively) (dimensionless). For the estimates of REE additions per unit area (g ha-1
year-1
) for limestone, gypsum and phosphate rock, equation 4 was used:
Where Aap is the simulated amount of phosphate rock, agricultural gypsum and limestone
applied to Brazilian soils (1.0, 3.0 and 4.0 ton ha-1
, respectively).
67
5.2.6 Statistical analyses
Mean, minimum, maximum and standard deviation values were calculated from the
data set. Pearson's linear correlation analysis (p < 0.05) was performed between the P2O5
levels and the sum of the REES in the fertilizers. Hierarchical grouping analysis (HGA) was
performed with the purpose of defining the similarity between the REEs distribution patterns
in fertilizers, carbonatites and phosphorites. Ward's algorithm and the measure of Euclidean
distance were used. The normalized values of REE in fertilizers plus phosphorites and
carbonitites were the variables used to define the groups. The entire statistical procedure was
performed using STATISTICA software (v.10.0).
5.3 Results and Discussion
5.3.1 Signatures and concentrations of REEs
Gypsum samples presented the lowest REEs concentrations, while the maximum
values were verified in OM + RP and SSP samples (Table 1). The variability total of REEs in
the samples followed the order Ce > La ≈ Pr > Nd > Sm > Dy > Ho ≈ Eu ≈ Sc ≈ Y > Yb > Gd
> Er ≈ Lu ≈ Tb.
Table 1. Range rare earth elements concentrations in correctives and P-containing fertilizers marketed in
Brazil
Lime stone Gypsum RP OM+RP SSP STP MAP
REEs --------------------------------------------- (mg kg-1
) -------------------------------------------------
Ce 1.3 – 9.7 0.2 – 3.3 3.5 – 85.1 1.1 – 950.8 6.3 – 1098.5 4.7 – 11.0 4.7 – 381.0
La 1.2 – 5.9 0.4 – 2.1 5.1 – 60.1 4.9 – 449.8 8.1 – 604.6 6.2 – 10.4 3.8 – 151.1
Nd <LQ – 6.7 <LQ – 0.5 28.3 – 45.4 0.3 – 426.0 3.4 – 442.9 2.2 – 3.5 0.2 – 164.9
Pr 1.1 – 2.1 0.3 – 0.8 1.7 – 14.4 0.7 – 119.4 1.2 – 135.3 1.3 – 2.6 0.3 – 46.5
Sm 1.8 – 3.1 0.5 – 1.2 3.5 – 9.0 1.2 – 65.4 0.7 – 61.0 0.9 – 25.3 0.9 – 25.3
Sc 0.1 – 1.3 0.1 – 0.6 0.4 – 5.5 1.2 – 20.4 1.5 – 18.8 2.0 – 2.8 0.7 – 18.4
Eu <LQ – 0.2 <LQ 1.6 – 2.2 0.3 – 23.7 0.2 – 14.6 0.1 – 0.3 0.4 – 9.7
Y 0.6 – 3.1 0.2 – 1.0 2.4 – 81.5 9.0 – 210.5 3.5 – 123.7 4.2 – 35.6 3.1 – 120.9
Dy 0.1 – 1.8 0.1 – 0.4 3.9 – 10.4 0.6 – 58.8 1.0 – 31.0 1.1 – 1.9 1.5 – 19.7
Gd 3.0 – 16.3 0.2 – 0.9 4.8 – 11.5 0.8 – 53.1 1.2 – 45.2 3.6 – 20.3 4.4 – 26.1
Er <LQ – 0.4 <LQ 1.2 – 5.0 0.3 – 17.1 0.3 – 7.7 0.1 – 1.5 2.4 – 7.9
Yb 0.1 – 0.3 <LQ – 0.1 0.2 – 5.1 0.5 – 9.7 0.4 – 5.7 0.4 – 2.2 0.3 – 10.3
Ho 0.2 – 0.3 0.1 – 0.2 0.3 – 2.0 0.2 – 8.4 0.1 – 4.7 0.3 – 0.5 0.3 – 3.3
Tb 0.9 – 2.0 0.5 – 0.8 0.7 – 2.0 0.3 – 8.6 0.1 – 6.6 1.3 – 1.6 0.5 – 3.5
Lu <LQ <LQ 0.1 – 0.8 0.1 – 1.5 0.1 – 0.9 0.1 – 0.4 0.1 – 2.0 RP rock phosphate, OM+RP organo-mineral + rock phosphate, SSP single superphosphate, STP triple superphosphate,
MAP monoammonium phosphate. LQ limit of quantification (0.013, 0.006, 0.007, 0.009 and 0.005 for Er, Eu, Nd, Yb
and Lu, respectively).
68
The OM + RP, SSP and MAP fertilizers had coefficient of variation (CV) ≥ 118% for
Ce, La, Nd, Pr, Sm, Y, Dy and Gd. This can be justified by the different sources of P used as
raw material in the manufacture of fertilizers. For this reason, the P sources of these fertilizers
were plotted (Figure 1).
Figure 1. Distribution pattern of rare earth elements in samples phosphate rock of igneous (A) and sedimentary
(B) origin. (C) Classification of phosphate fertilizers as origin. RP phosphate rock, OM+RP organo-mineral +
phosphate rock, SSP single superphosphate, ST triple superphosphate, MAP monoammonium phosphate, Phosp.
A-D phosphorite samples, Carb. A-D carbonatite samples, REE rare earth elements, PAAS Post Archaen
Australian Shale (McLennan 2001), *normalized values with PAAS (for example: Cesample/CePAAS). a Cerva-
Alves et al. 2017; b Poletti et al. 2016; c Trofanenko et al. 2016; d Ramos et al. 2016; e Garnit et al. 2017; f Xin
et al. 2016; g Khater et al. 2016; h Hein et al. 2016.
The normalized REEs values of carbonatites and phosphorites were graphically plotted
to visualize their respective distribution patterns (Figure 1A - B). REEs signatures were
distinct and well defined for each source of phosphorus. Fertilizers produced from
carbonatites are more enriched in REEs than those from phosphorites (OTERO et al., 2005;
RAMOS et al., 2016). However, carbonatites of the calcitic and dolomitic type presented
lower levels, mainly for La, Ce, Pr and Nd, which can be reflected in the fertilizers produced
from these sources.
69
Based on the results of the hierarchical cluster analysis, 77% (n = 23) of the fertilizers
were grouped as sedimentary origin (phosphorite derivatives) (Figure 1C). Around 90% of the
Brazilian phosphate reserve is igneous and a few sedimentary reserves are active in the
country (ABRAM et al., 2011); on the other hand, Brazil imports about 50% of the phosphate
fertilizers used in the country. The imported amount of phosphate rock and MAP in 2014 was
1.7 million tonnes (IFA, 2016). The phosphate reserves of the country that exported these
fertilizers (Morocco) are predominantly sedimentary (KRATZ; SCHICK; SCHNUG, 2016;
CÁNOVAS et al., 2017).
In general, the fertilizers derived from carbonatite had ΣREEs higher than fertilizers
derived from phosphorite. The ΣREEs in OM + RP and SSP of igneous origin were 5 to 8
times larger, respectively, as compared to the ones of sedimentary origin (Figure 2A - B). A
similar result was obtained by Ramos et al. (2016), which evaluated REEs in phosphate
concentrates and fertilizers derived from carbonatites and phosphorites. Other studies have
shown that apatite carbonatite minerals are more enriched in REEs than phosphate apatites
(FLEISCHER; ALTSCHULER, 1986; LAPIDO-LOUREIRO; FIGUEIREIDO; TOREZAN,
1989; HUGHES; CAMERON; MARIANO, 1991).
The OM + RP and SSP sources presented the highest ΣREEs, while the sources
concentrated in P2O5 posed the lowest ones. Significant and negative correlation between
ΣREE and P2O5 levels was observed for fertilizers, either sedimentary or igneous origin
(Figure 2 C - D). Turra, Fernandes and Bacchi (2011) evaluating REEs concentrations in
agricultural inputs found the lowest ΣREEs (12.0 mg kg-1
) for MAP among phosphate
sources. Ramos et al. (2016) found that ΣREEs in TSP and MAP samples was 57% and 233%
lower than the phosphate concentrate that gave rise to them, respectively. The industrial
processes of purification of these sources are the probable causes of reduction in REEs
(TURRA; FERNANDES; BACCHI, 2011; RAMOS et al., 2016).
Regardless of origin, fertilizers had a higher accumulation of light REEs (Light ΣREE
- Ce, La, Nd, Pr, Sm, Sc and Eu) in relation to the heavy ones (ΣREE - Y, Dy, Gd, Er, Yb,
Ho, Tb and Lu), except for MAP and TSP. The mean accumulation of light REEs was 154
and 660% higher than the heavy REEs in phosphorus and carbonatite derived fertilizers,
respectively. Light REEs are more abundant in the crust than the heavy REEs (VONCKEN,
2016) and the depletion of the light REEs in MAP and TSP may be related to the greater
solubility of these elements in H3PO4 and their possible loss in the washing process during
fertilizers production.
70
Figure 2. Sum (± standard error) of light rare earth elements (ƩLREE), heavy rare earth elements (ƩHREE) and
total (ƩREE); linear correlation between P2O5 and rare earth elements in containing-P fertilizers marketed in
Brazil. r correlation coefficient, ne not evaluated, p < 0.01 significant effect at probability level of 99%. OM+RP
organo-mineral + rock phosphate, SSP single superphosphate, MAP monoammonium phosphate, STP triple
superphosphate, RP rock phosphate
The mean ΣREEs in limestone and gypsum were 27.1 (± 6.6) and 4.6 (± 2.6) mg kg-1
,
respectively, i.e. they were much lower than fertilizers. The ΣREE-light / ΣREE-heavy ratio
was 1.12 (limestone) and 0.31 (gypsum). In carbonated rocks, the ΣREEs can vary from 21.4 -
52.0 mg kg-1
(KABATA-PENDIAS, 2011).
The average REEs content in the fertilizers derived from carbonatite was higher than
the phosphorite derivatives (Figure 3). The mean contents of the light REEs followed the
order Ce > La > Nd > Pr > Sm > Sc > Eu. For the heavy REEs, the order was Y > Gd > Dy >
Er > Yb > Tb > Ho > Lu.
The highest concentrations for Ce, La, Nd, Pr and Y were observed in the OM + RP,
SSP and MAP sources derived from carbonatite. The TSP was the fertilizer with the lowest
levels. Mean Ce concentrations in SSP, OM + RP and MAP originating from carbonatite
differed by 1362%, 660% and 348% relatively to phosphorite, respectively. Among fertilizers
derived from phosphorite, TSP and MAP were the ones that had the highest levels of Gd.
Phosphate rocks derived phosphorite had mean values of heavy REEs higher than those of
carbonatite.
71
Figure 3. Heatmap showing means ± standard deviation of rare earth elements (REE) contents (mg kg-1) for
samples fertilizers P-containing (n = 30) marketed in Brazil. S1 triple superphosphate (sedimentary), S2 single
superphosphate (sedimentary), S3 phosphate rock (sedimentary), S4 phosphate rock (igneous), S5
monoammonium phosphate (sedimentary), S6 organo-mineral + phosphate rock (sedimentary), S7
monoammonium phosphate (igneous), S8 single superphosphate (igneous), S9 organo-mineral + phosphate rock
(igneous). Within each line, darker colors indicate higher REE contents
The REEs contents of the present study were compared with those reported in the
literature (Table 2). The average contents of Ce, La, Nd and Y in the fertilizers produced from
the Catalão and Araxá ores in southeastern Brazil were the highest cited in the literature.
The mean REEs concentraions reported here (of igneous origin) were among the
lowest. The REEs in the fertilizers produced from the Catalão ore were 2 to 7 times higher
than the results obtained in the present study. When comparing with other studies in Brazil,
our SSP samples presented similar Ce concentrations, while the La and Nd concentrations in
72
our study were lower than reported elesewhere. On the other hand, the mean REE
concentrations in the MAP samples we measured were higher than other studies. Regarding
fertilizers of sedimentary origin, the average REEs found in Brazilian fertilizers were the
highest.
Mean (mg kg-1
) in the limestone samples followed the order Gd (8.1 ± 3.1) > Ce (5.1 ±
2.2) > La (3.7 ± 1.2) > Sm (2.5 ± 0.4) > Y (2.0 ± 0.6) > Nd (1.6 ± 0.8) ≈ Pr (1.6 ± 0.3) > Tb
(1.4 ± 0.3) > Dy (0.8 ± 0.5) > Sc (0.6 ± 0.3) > Ho (0.2 ± 0.1) ≈ Yb (0.2 ± 0.1). Limestone
samples analyzed by Turra, Fernandes and Bacchi (2011) presented mean concentrations of
15.0, 7.5, 7.3 and 1.4 mg kg-1
of Ce, Nd, La and Sc, respectively, i.e., higher than recorded in
our study. Mean concentrations for gypsum were the lowest among all evaluated sources and
presented the following order: Ce (1.6 ± 1.2) > La (1.2 ± 0.7) > Sm (0.9 ± 0.2) > Tb (0.7 ±
0.1) > Y , 6 ± 0.3) > Pr (0.5 ± 0.2) ≈ Gd (0.5 ± 0.2) > Nd (0.4 ± 0.2) > Sc (0.3 ± 0.2) ) > Dy
(0.2 ± 0.1) > Ho (0.1 ± 0.02) ≈ Yb (0.1 ± 0.02). Erbium, Lu and Eu had no detectable levels
neither in limestone nor gypsum samples.
5.3.2 REES solubility in P-containing fertilizers
Table 2. Comparison of rare earth elements contents in different phosphate fertilizers report in literature
Fertilizer Locality Origin
Ce La Nd Pr Y
--------------------- (mg kg-1
) ---------------------
SSP Catalão/Brazil1
igneous 3934.0 1926.0 1529.0 447.0 122.0
SSP Araxá/Brazil1
igneous 1866.0 895.0 899.0 235.0 322.0
MAP Tapira/Brazil1
igneous 809.0 399.0 373.0 96.0 168.0
Mixed (12-12-17) Spanish2
igneous 600.0 500.0 181.0 na na
Mixed (15-15-17) Spanish2
igneous 744.0 619.0 214.0 na na
SSP Brazil7
igneous 512.2 264.5 213.9 62.8 56.9
MAP Brazil7
igneous 381.0 151.1 164.9 46.5 120.9
SSP Egypt3
sedimentary 59.8 na na na na
SSP Pakistan3
sedimentary 40.2 18.4 na na na
Mixed (10-10-18) Spanish2
sedimentary 8.0 14.1 7.0 na na
Mixed (7-14-16) Spanish2
sedimentary 16.0 32.4 16.0 na na
SSP Egypt4
sedimentary 8.5 18.0 na na na
SSP Brazil7
sedimentary 35.2 27.4 16.6 5.5 41.3
MAP Brazil7
sedimentary 85.5 38.9 37.2 10.3 78.9
MAP Brazil5
na 1.0 7.3 2.4 na na
SSP Brazil5
na 673.0 1499.0 770.0 na na
Mixed na6
na 65.1 44.4 32.6 8.5 66.2
SSP single superphosphate, MAP monoammonium phosphate, Mixed (N – P – K) and na not available, 1Ramos et al.
(2016), 2Otero et al. (2005), 3Waheed et al. (2011), 4Abdel et al. (2001), 5Turra et al. (2011), 6Hu et al. (1998), 7Present
study
73
The data of our study are the first records on the solubility of REE in phosphate
fertilizers in Brazil. In general, fertilizers derived from phosphorites presented the greatest
solubilities for REEs in relation to fertilizers derivated from carbonatite, the mean solubilities
were 25% and 242% higher for heavy REEs and light REEs, respectively. The phosphorite-
derived MAP was the fertilizer that had the highest percentages of soluble REEs. However,
the solubilities of Y, Er and Ho were higher in the carbonatite-derived MAP fertilizer. The
lowest solubilities were observed in the RP and STP sources (Figure 4).
Figure 4. Average solubility rare earth elements in fertilizers containing-P of igneous and sedimentary origin,
marketed in Brazil. SSP single superphosphate, STP triple superphosphate, RP rock phosphate, OM+RP organo-
mineral + rock phosphate, MAP monoammonium phosphate
74
The organo-mineral fertilizers + RP derived from carbonatite, which had the highest
levels of REEs, had average solubility for light REEs of 15% and 30% for heavy REEs. SSP
fertilizers derived from phosphorite, the most consumed in Brazil, had average solubilities of
52% and 50% for light REEs and heavy REEs, respectively.
Ramos et al. (2016) evaluating the mineralogical assemblage of different phosphate
concentrates and fertilizers by X-ray diffraction using synchrotron radiation, obtained the
following results: (i) apatite, carbonate-flouropatite and carbonate-hydroxyapatite were the
main mineral phases containing REEs; (ii) samples derived from carbonatite had higher
diversity of minerals containing REEs than sedimentary phosphorites; (iii) phosphate
fertilizers had greater diversity of minerals containing REEs than the raw materials that
originated them. Thus, it is reasonable to assume that (i) the solubility of REEs is directly
related to the solubility of phosphates; (ii) that in the process of manufacturing phosphate
fertilizers, the step of wet digestion with sulfuric acid on the phosphate concentrate can
promote the dissolution of mineral phases and the neogenesis of others.
Thus, the low solubility of REEs in the carbonatite-derived samples may be explained
by the following reasons: (i) the carbonatite phosphate has higher Fe values than the
sedimentary phosphate (KRATZ; SCHICK; SCHNUG, 2016); (ii) REES in fertilizers of
igneous origin can co-precipitate as Fe/Al phosphate minerals in the industrial stage of wet
digestion (ex.: wavellite [Al3(OH, F)3(PO4)2.5H2O]; childrenite [(Fe, Mn)AlPO4(OH)2.H2O];
lazurite [(Mg, Fe)Al2(PO4)2(OH)2]); and (iii) iron and aluminum phosphates have low
recoveries (4 - 17%) for the 2% citric acid extraction relatively to the total concentration
(ALCARDE; PONCHIO, 1979).
5.3.3 Potential environmental risk and REE additions to Brazilian soils
The REEs concentrations in the phosphate fertilizers were expressed as a function of
the percentage of P2O5 in the samples (Figure 5A - B) in similarity with what is done by the
Brazilian legislation for the investigation of heavy metals of environmental importance (Cd,
Pb and Cr) (MAPA, 2006).
75
Figure 5. Rare earth elements concentration (± standard error) per % P2O5 in phosphate fertilizers marketed in
Brazil (A-B). Mean values (± standard error) of total (C-D) and per unit area additions (E-F) of rare earth
elements in agricultural soils via phosphate fertilizers, lime stone and gypsum marketed in Brazil. LREE light
rare earth elements, HREE high rare earth elements, OM+RP organo-mineral + rock phosphate, SSP single
superphosphate, MAP monoammonium phosphate, STP triple superphosphate, R. phosphate rock phosphate. The
columns of red color are the most relevant results for the context of soil environmental pollution
76
There are no regulatory values for REEs in agricultural inputs in Brazil. Therefore, we
assumed the maximum permissible value of 40 mg kg-1
/% P2O5 (the same for Cr in the
legislation) to investigate the potential environmental risk of phosphate fertilizers in
contributing REEs to soils. This decision was based on the following aspects: (i) Cr as well as
REEs is classified geochemically as a lithophilic elements; (ii) in the environment, Cr and
REEs commonly have valence +3 (KABATA-PENDIAS, 2011), and; (iii) since information
on the deleterious effects of environmental exposure on REE in agricultural soils is scarce,
adopting a less conservative value is an environmentally and economically viable decision.
Mean values higher than 40 mg kg-1
/% P2O5 were verified for Ce, La, Nd and Y in
fertilizers SSP, OM + RP, RP (all derived from carbonatite) and MAP (from both sources).
Among the values of 20-40 mg kg-1
/% P2O5, OM + RPphosphorite (Ce, La and Pr), RPphosphorite
(Ce, La, Pr and Y), OM + RPcarbonatite (La, Nd and Y) and RPcarbonatite (La, Nd and Y).
However, 84% of the cases had levels < 20 mg kg-1
/% P2O5. The concentrations of Ce, La, Nd
and Y in the SSP, regardless of origin, were higher than those of Cd, Cr and Pb registered in
phosphate fertilizers marketed in Brazil (CAMPOS et al., 2005; SILVA; NASCIMENTO;
ARAÚJO, 2017).
Mean additions (g ha-1
) of La, Nd and Ce by OM + RP and SPP (both carbonatite
derivatives) were ≥ 329%, 336% and 405% higher, respectively, than the other sources. For
light REEs, 65% of the cases had estimated additions ≤ 10 g ha-1
(Figure 5 E).
In general, OM + RPcarbonatite had the highest additions (g ha-1
) for heavy REEs. The
additions of Y and Dy were ≥ 157 and 220% higher, respectively, than the other sources.
Yttrium added by the SSPcarbonatite and RPphosphorite were higher than 40 g ha-1
. The limestone
added a significant amount of Gd compared to the other sources (Figure 5F).
According to the estimates, the elements Ce, La, Nd and Y had the highest annual
contributions to soils (> 1800 t year-1) (Figure 5 C-D). The OM + RP and SSP sources (both
carbonatite-derived) plus MAP (from both origins) accounted for more than 200 t y-1
of
yttrium. The limestone annually added 96% of the total estimated Gd. SSP and MAP, both
derived from carbonatite, were the most enriched fertilizers in REEs (Figure 3). They
contributed annually 6438 and 3678 t year-1 REEs, respectively.
5.4 Conclusions
Phosphate fertilizers are more enriched in REEs than soil correctives. The levels of
REEs in phosphate fertilizers vary depending on the raw material used in the industry and
77
industrial processes related to the manufacture of concentrated fertilizers. The geochemical
signature of REEs associated with the multivariate analysis are useful tools to trace the origin
of the raw material used in the phosphate fertilizer industry.
Light REEs were more abundant than heavy REES in both the phosphate fertilizers
and corrective samples. Fertilizers derived from carbonatites (igneous origin) have the largest
accumulation of REEs, especially SSP which is the most consumed fertilizer in Brazil.
However, the solubility of REEs in fertilizers was higher in those derived from phosphorite
(sedimentary origin). The low solubility (< 20%) of REEs in phosphate fertilizers of igneous
origin may be associated with the co-precipitation process with Fe/Al phosphates that are
newly formed in the industrial stages of wet digestion with sulfuric acid.
The addition of REEs by the continuous application of high doses of P fertilizers and
soil correctives to Brazilian soils may represent a potential risk to agricultural and
environmental sustainability, since 13,000 tons of REEs are incorporated annually to soils.
The SSP derived from carbonatite annually contribute with over 6000 t year-1
of REEs, with
the highest additions being recorded for Ce, La, Nd and Y (512, 242, 229, 113 g ha-1
,
respectively). The limestone contributed 250 metric tons of Gd, more than any other source,
since Brazilian agriculture consumes more than 33 million tons of this input annually. Thus,
studies on bioavailability, ecotoxicity and the definition of quality reference values for REEs
in soils, especially for Ce, La, Nd and Y, are important for environmental safety.
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6. FINAL CONSIDERATIONS
Soils contaminated with heavy metals and rare earth elements pose a potential risk to
human health and ecosystem functionality, therefore, monitoring studies of these elements in
soils and also identifying possible sources of contamination are necessary for the safety of
public and environmental health. In the agricultural scenario, metals and rare earths contained
in the soil can be transferred to the food chain and represent risk, and phosphate and
corrective fertilizers are two important sources of soil contamination by these elements.
In this context, the study evaluated the concentrations of heavy metals in phosphate
fertilizers used in Brazilian agriculture, and although the concentration of the metals in the
fertilizers were not above the allowable limits, it was observed that the phosphate fertilization
for a long time in cultivated soils was cause of the enrichment with metals, mainly Cd. All
evaluated soils have been contaminated by Cd and this metal is mobile in the soil. The
transfer of Cd to the biosphere and/or surface and subsurface waters is an important issue to
be discussed. Future studies evaluating the content of this element in water resources inserted
in the context of sugarcane monoculture are recommended because drinking water as well as
fish and other animals may become significant routes of exposure for the rural population.
With regard to soil polluted with Cd, which had concentrations > 3.0 mg kg-1
, more
detailed descriptions are necessary to judge the real risk. Studies on the compartmentalization
of this metal in the soil, the human bioaccessible fraction and a characterization of the routes
of exposure are necessary for the elaboration of action plans.
In relation to the rare earth elements that are another group of contaminants of great
environmental interest, but that are less studied. Brazilian phosphate fertilizers and correctives
have been shown to be significant sources for soils, mainly for Ce, La, Nd, Gd and Y. There
is no regulation for rare earths in soil and not in agricultural inputs. Little is known about the
potential deleterious effects of these elements for the functionality of an agroecosystem.
Ecotoxicological studies and risk assessment of human health by exposure to rare earths are
necessary for the derivation of maximum permissible concentrations. The bioavailability of
these elements in the soil after phosphate fertilization is another important issue to be
addressed, since it would elucidate the ability of these contaminants to be transferred to other
spheres of the environment.